build-system: Rewrite using gexps.

* guix/packages.scm (expand-input): Remove 'store', 'system', and
  'cross-system' parameters; add #:native?.  Rewrite to return
  name/gexp-input tuples.
  (bag->derivation): Adjust accordingly.  Lower (bag-build bag).
  (bag->cross-derivation): Ditto.  Instead of #:native-drvs and
  #:target-drvs, pass #:build-inputs, #:host-inputs, and #:target-inputs.
  (%derivation-cache): Remove.
* gnu/packages/bootstrap.scm (raw-build): Turn into a monadic procedure.
* gnu/packages/commencement.scm (glibc-final)[arguments]: Use
  'gexp-input' for the #:allowed-references argument.
* guix/build-system/cmake.scm (cmake-build): Remove 'store' parameter.
  Switch to the use of gexps and 'gexp->derivation'.
  (lower): Remove #:source from 'private-keywords'.
* guix/build-system/glib-or-gtk.scm (glib-or-gtk-build, lower):
  Likewise.
* guix/build-system/font.scm (font-build): Likewise.
* guix/build-system/gnu.scm (gnu-build): Likewise, and remove
  'canonicalize-reference'.
  (gnu-cross-build): Likewise, and expect #:build-inputs, #:host-inputs,
  and #:target-inputs instead of #:native-drvs and #:target-drvs.
  (lower): Likewise.
* guix/build-system/perl.scm (perl-build, lower): Likewise.
* guix/build-system/python.scm (python-build, lower): Likewise.
* guix/build-system/ruby.scm (ruby-build, lower): Likewise.
* guix/build-system/waf.scm (waf-build, lower): Likewise.
* guix/build-system/trivial.scm (guile-for-build): Remove.
  (trivial-build): Remove 'store' parameter, change to gexps.
  (trivial-cross-build): Ditto, and change to #:build-inputs & co.
* guix/build-system/cargo.scm (cargo-build): Change to 'gexp->derivation'.
* guix/build-system/copy.scm (copy-build): Likewise.
* guix/build-system/dune.scm (dune-build): Likewise.
* guix/build-system/guile.scm (guile-build, guile-cross-build):
  Likewise.
* guix/build-system/meson.scm (meson-build): Likewise.
* guix/build-system/ocaml.scm (ocaml-build): Likewise.
* guix/build-system/scons.scm (scons-build): Likewise.
* guix/build-system/texlive.scm (texlive-build): Likewise.
* guix/build-system/android-ndk.scm (android-ndk-build): Likewise.
* guix/build-system/ant.scm (ant-build): Likewise.
* guix/build-system/asdf.scm (asdf-build/source, asdf-build): Likewise.
* guix/build-system/chicken.scm (chicken-build): Likewise.
* guix/build-system/clojure.scm (clojure-build): Likewise.
(source->output-path, maybe-guile->guile): Remove.
* guix/build-system/dub.scm (dub-build): Likewise.
* guix/build-system/emacs.scm (emacs-build): Likewise.
* guix/build-system/go.scm (go-build): Likewise.
* guix/build-system/haskell.scm (haskell-build): Likewise.
* guix/build-system/julia.scm (julia-build): Likewise.
* guix/build-system/linux-module.scm (linux-module-build)
(linux-module-build-cross): Likewise.
* guix/build-system/maven.scm (maven-build): Likewise.
* guix/build-system/minify.scm (minify-build): Likewise.
* guix/build-system/node.scm (node-build): Likewise.
* guix/build-system/qt.scm (qt-build, qt-cross-build): Likewise.
* guix/build-system/r.scm (r-build): Likewise.
* guix/build-system/rakudo.scm (rakudo-build): Likewise.
* guix/build-system/renpy.scm (renpy-build): Likewise.
* tests/builders.scm ("gnu-build"): Call 'store-lower' on 'gnu-build'.
  Pass #:source parameter.
* tests/packages.scm ("search paths"): Use 'abort-to-prompt' instead of
  a normal return from the 'build' method.
  ("package->bag, sensitivity to %current-target-system"): Change 'build'
  to match the new build system signature.

squash! build-system: Rewrite using gexps.

squash! build-system: Rewrite using gexps.

43 files changed, 1541 insertions, 2160 deletions

diff --git a/.dir-locals.el b/.dir-locals.el
index 8f07a08eb5..378071ea67 100644
--- a/.dir-locals.el
+++ b/.dir-locals.el
@@ -119,6 +119,7 @@
    (eval . (put 'with-extensions 'scheme-indent-function 1))
    (eval . (put 'with-parameters 'scheme-indent-function 1))
    (eval . (put 'let-system 'scheme-indent-function 1))
+   (eval . (put 'with-build-variables 'scheme-indent-function 2))
 
    (eval . (put 'with-database 'scheme-indent-function 2))
    (eval . (put 'call-with-database 'scheme-indent-function 1))
diff --git a/gnu/packages/bootstrap.scm b/gnu/packages/bootstrap.scm
index c50e94e891..c8844a40a8 100644
--- a/gnu/packages/bootstrap.scm
+++ b/gnu/packages/bootstrap.scm
@@ -32,11 +32,13 @@
   #:use-module (guix build-system gnu)
   #:use-module (guix build-system trivial)
   #:use-module ((guix store)
-                #:select (run-with-store add-to-store add-text-to-store))
+                #:select (%store-monad interned-file text-file store-lift))
   #:use-module ((guix derivations)
-                #:select (derivation derivation-input derivation->output-path))
-  #:use-module ((guix utils) #:select (gnu-triplet->nix-system))
+                #:select (raw-derivation derivation-input derivation->output-path))
+  #:use-module (guix utils)
+  #:use-module ((guix build utils) #:select (elf-file?))
   #:use-module ((guix gexp) #:select (lower-object))
+  #:use-module (guix monads)
   #:use-module (guix memoization)
   #:use-module (guix i18n)
   #:use-module (srfi srfi-1)
@@ -376,59 +378,58 @@ or false to signal an error."
               %bootstrap-base-urls))
     (sha256 (bootstrap-guile-hash system))))
 
-(define (download-bootstrap-guile store system)
+(define (download-bootstrap-guile system)
   "Return a derivation that downloads the bootstrap Guile tarball for SYSTEM."
   (let* ((path (bootstrap-guile-url-path system))
          (base (basename path))
          (urls (map (cut string-append <> path) %bootstrap-base-urls)))
-    (run-with-store store
-      (url-fetch urls 'sha256 (bootstrap-guile-hash system)
-                 #:system system))))
+    (url-fetch urls 'sha256 (bootstrap-guile-hash system)
+               #:system system)))
 
-(define* (raw-build store name inputs
+(define* (raw-build name inputs
                     #:key outputs system search-paths
                     #:allow-other-keys)
   (define (->store file)
-    (run-with-store store
-      (lower-object (bootstrap-executable file system)
-                    system)))
-
-  (let* ((tar   (->store "tar"))
-         (xz    (->store "xz"))
-         (mkdir (->store "mkdir"))
-         (bash  (->store "bash"))
-         (guile (download-bootstrap-guile store system))
-         ;; The following code, run by the bootstrap guile after it is
-         ;; unpacked, creates a wrapper for itself to set its load path.
-         ;; This replaces the previous non-portable method based on
-         ;; reading the /proc/self/exe symlink.
-         (make-guile-wrapper
-          '(begin
-             (use-modules (ice-9 match))
-             (match (command-line)
-               ((_ out bash)
-                (let ((bin-dir    (string-append out "/bin"))
-                      (guile      (string-append out "/bin/guile"))
-                      (guile-real (string-append out "/bin/.guile-real"))
-                      ;; We must avoid using a bare dollar sign in this code,
-                      ;; because it would be interpreted by the shell.
-                      (dollar     (string (integer->char 36))))
-                  (chmod bin-dir #o755)
-                  (rename-file guile guile-real)
-                  (call-with-output-file guile
-                    (lambda (p)
-                      (format p "\
+    (lower-object (bootstrap-executable file system)
+                  system))
+
+  (define (make-guile-wrapper bash guile-real)
+    ;; The following code, run by the bootstrap guile after it is unpacked,
+    ;; creates a wrapper for itself to set its load path.  This replaces the
+    ;; previous non-portable method based on reading the /proc/self/exe
+    ;; symlink.
+    '(begin
+       (use-modules (ice-9 match))
+       (match (command-line)
+         ((_ out bash)
+          (let ((bin-dir    (string-append out "/bin"))
+                (guile      (string-append out "/bin/guile"))
+                (guile-real (string-append out "/bin/.guile-real"))
+                ;; We must avoid using a bare dollar sign in this code,
+                ;; because it would be interpreted by the shell.
+                (dollar     (string (integer->char 36))))
+            (chmod bin-dir #o755)
+            (rename-file guile guile-real)
+            (call-with-output-file guile
+              (lambda (p)
+                (format p "\
 #!~a
 export GUILE_SYSTEM_PATH=~a/share/guile/2.0
 export GUILE_SYSTEM_COMPILED_PATH=~a/lib/guile/2.0/ccache
 exec -a \"~a0\" ~a \"~a@\"\n"
-                              bash out out dollar guile-real dollar)))
-                  (chmod guile   #o555)
-                  (chmod bin-dir #o555))))))
-         (builder
-          (add-text-to-store store
-                             "build-bootstrap-guile.sh"
-                             (format #f "
+                        bash out out dollar guile-real dollar)))
+            (chmod guile   #o555)
+            (chmod bin-dir #o555))))))
+
+  (mlet* %store-monad ((tar   (->store "tar"))
+                       (xz    (->store "xz"))
+                       (mkdir (->store "mkdir"))
+                       (bash  (->store "bash"))
+                       (guile (download-bootstrap-guile system))
+                       (wrapper -> (make-guile-wrapper bash guile))
+                       (builder
+                        (text-file "build-bootstrap-guile.sh"
+                                   (format #f "
 echo \"unpacking bootstrap Guile to '$out'...\"
 ~a $out
 cd $out
@@ -441,19 +442,19 @@ $out/bin/guile -c ~s $out ~a
 
 # Sanity check.
 $out/bin/guile --version~%"
-                                     (derivation->output-path mkdir)
-                                     (derivation->output-path xz)
-                                     (derivation->output-path tar)
-                                     (format #f "~s" make-guile-wrapper)
-                                     (derivation->output-path bash)))))
-    (derivation store name
-                (derivation->output-path bash) `(,builder)
-                #:system system
-                #:inputs (map derivation-input
-                              (list bash mkdir tar xz guile))
-                #:sources (list builder)
-                #:env-vars `(("GUILE_TARBALL"
-                              . ,(derivation->output-path guile))))))
+                                           (derivation->output-path mkdir)
+                                           (derivation->output-path xz)
+                                           (derivation->output-path tar)
+                                           (object->string wrapper)
+                                           (derivation->output-path bash)))))
+    (raw-derivation name
+                    (derivation->output-path bash) `(,builder)
+                    #:system system
+                    #:inputs (map derivation-input
+                                  (list bash mkdir tar xz guile))
+                    #:sources (list builder)
+                    #:env-vars `(("GUILE_TARBALL"
+                                  . ,(derivation->output-path guile))))))
 
 (define* (make-raw-bag name
                        #:key source inputs native-inputs outputs
diff --git a/gnu/packages/commencement.scm b/gnu/packages/commencement.scm
index 7c39a84008..3eba960447 100644
--- a/gnu/packages/commencement.scm
+++ b/gnu/packages/commencement.scm
@@ -52,6 +52,7 @@
   #:use-module (gnu packages pkg-config)
   #:use-module (gnu packages rsync)
   #:use-module (gnu packages xml)
+  #:use-module (guix gexp)
   #:use-module (guix packages)
   #:use-module (guix download)
   #:use-module (guix build-system gnu)
@@ -3375,7 +3376,7 @@ exec ~a/bin/~a-~a -B~a/lib -Wl,-dynamic-linker -Wl,~a/~a \"$@\"~%"
    ;; if 'allowed-references' were per-output.
    (arguments
     `(#:allowed-references
-      ((,gcc-boot0 "lib")
+      (,(gexp-input gcc-boot0 "lib")
        ,(kernel-headers-boot0)
        ,static-bash-for-glibc
        ,@(if (hurd-system?)
diff --git a/guix/build-system/android-ndk.scm b/guix/build-system/android-ndk.scm
index dbfa626a19..d8896e2305 100644
--- a/guix/build-system/android-ndk.scm
+++ b/guix/build-system/android-ndk.scm
@@ -1,5 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2016 Danny Milosavljevic <dannym@scratchpost.org>
+;;; Copyright © 2021 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -20,7 +21,8 @@
   #:use-module (guix search-paths)
   #:use-module (guix store)
   #:use-module (guix utils)
-  #:use-module (guix derivations)
+  #:use-module (guix gexp)
+  #:use-module (guix monads)
   #:use-module (guix packages)
   #:use-module (guix build-system)
   #:use-module (guix build-system gnu)
@@ -34,62 +36,49 @@
     (guix build syscalls)
     ,@%gnu-build-system-modules))
 
-(define* (android-ndk-build store name inputs
-                      #:key
-                      (tests? #t)
-                      (test-target #f)
-                      (phases '(@ (guix build android-ndk-build-system)
-                                  %standard-phases))
-                      (outputs '("out"))
-                      (make-flags ''())
-                      (search-paths '())
-                      (system (%current-system))
-                      (guile #f)
-                      (imported-modules %android-ndk-build-system-modules)
-                      (modules '((guix build android-ndk-build-system)
-                                 (guix build utils))))
+(define* (android-ndk-build name inputs
+                            #:key
+                            source
+                            (tests? #t)
+                            (test-target #f)
+                            (phases '(@ (guix build android-ndk-build-system)
+                                        %standard-phases))
+                            (outputs '("out"))
+                            (make-flags #~'())
+                            (search-paths '())
+                            (system (%current-system))
+                            (guile #f)
+                            (imported-modules %android-ndk-build-system-modules)
+                            (modules '((guix build android-ndk-build-system)
+                                       (guix build utils))))
   "Build SOURCE using Android NDK, and with INPUTS."
   (define builder
-    `(begin
-       (use-modules ,@modules)
-       (android-ndk-build #:name ,name
-                    #:source ,(match (assoc-ref inputs "source")
-                                (((? derivation? source))
-                                 (derivation->output-path source))
-                                ((source)
-                                 source)
-                                (source
-                                 source))
-                    #:system ,system
-                    #:test-target ,test-target
-                    #:tests? ,tests?
-                    #:phases ,phases
-                    #:make-flags (cons* "-f"
-                                         ,(string-append
-                                          (derivation->output-path
-                                           (car (assoc-ref inputs "android-build")))
-                                          "/share/android/build/core/main.mk")
-                                         ,make-flags)
-                    #:outputs %outputs
-                    #:search-paths ',(map search-path-specification->sexp
-                                          search-paths)
-                    #:inputs %build-inputs)))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
 
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
+          (android-ndk-build #:name #$name
+                             #:source #+source
+                             #:system #$system
+                             #:test-target #$test-target
+                             #:tests? #$tests?
+                             #:phases #$phases
+                             #:make-flags
+                             (cons* "-f"
+                                    #$(file-append (car (assoc-ref inputs
+                                                                   "android-build"))
+                                                   "/share/android/build/core/main.mk")
+                                    #$make-flags)
+                             #:outputs #$(outputs->gexp outputs)
+                             #:search-paths '#$(map search-path-specification->sexp
+                                                    search-paths)
+                             #:inputs #$(input-tuples->gexp inputs)))))
 
-  (build-expression->derivation store name builder
-                                #:inputs inputs
-                                #:system system
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build))
+  (mlet %store-monad  ((guile (package->derivation (or guile (default-guile))
+                                                   system #:graft? #f)))
+    (gexp->derivation name builder
+                      #:system system
+                      #:guile-for-build guile)))
 
 (define* (lower name
                 #:key source inputs native-inputs outputs system target
@@ -98,7 +87,7 @@
   "Return a bag for NAME."
 
   (define private-keywords
-    '(#:source #:target #:inputs #:native-inputs #:outputs))
+    '(#:target #:inputs #:native-inputs #:outputs))
 
   (and (not target) ;; TODO: support cross-compilation
        (bag
diff --git a/guix/build-system/ant.scm b/guix/build-system/ant.scm
index 1809d1f3d2..cb48c4226c 100644
--- a/guix/build-system/ant.scm
+++ b/guix/build-system/ant.scm
@@ -1,5 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2016 Ricardo Wurmus <rekado@elephly.net>
+;;; Copyright © 2021 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -20,7 +21,8 @@
   #:use-module (guix store)
   #:use-module (guix utils)
   #:use-module (guix packages)
-  #:use-module (guix derivations)
+  #:use-module (guix gexp)
+  #:use-module (guix monads)
   #:use-module (guix search-paths)
   #:use-module (guix build-system)
   #:use-module (guix build-system gnu)
@@ -73,7 +75,7 @@
                 #:rest arguments)
   "Return a bag for NAME."
   (define private-keywords
-    '(#:source #:target #:jdk #:ant #:zip #:inputs #:native-inputs))
+    '(#:target #:jdk #:ant #:zip #:inputs #:native-inputs))
 
   (and (not target)                               ;XXX: no cross-compilation
        (bag
@@ -94,8 +96,9 @@
          (build ant-build)
          (arguments (strip-keyword-arguments private-keywords arguments)))))
 
-(define* (ant-build store name inputs
+(define* (ant-build name inputs
                     #:key
+                    source
                     (tests? #t)
                     (test-target "check")
                     (configure-flags ''())
@@ -119,49 +122,34 @@
                                (guix build utils))))
   "Build SOURCE with INPUTS."
   (define builder
-    `(begin
-       (use-modules ,@modules)
-       (ant-build #:name ,name
-                  #:source ,(match (assoc-ref inputs "source")
-                              (((? derivation? source))
-                               (derivation->output-path source))
-                              ((source)
-                               source)
-                              (source
-                               source))
-                  #:make-flags ,make-flags
-                  #:configure-flags ,configure-flags
-                  #:system ,system
-                  #:tests? ,tests?
-                  #:test-target ,test-target
-                  #:build-target ,build-target
-                  #:jar-name ,jar-name
-                  #:main-class ,main-class
-                  #:test-include (list ,@test-include)
-                  #:test-exclude (list ,@test-exclude)
-                  #:source-dir ,source-dir
-                  #:test-dir ,test-dir
-                  #:phases ,phases
-                  #:outputs %outputs
-                  #:search-paths ',(map search-path-specification->sexp
-                                        search-paths)
-                  #:inputs %build-inputs)))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
+          (ant-build #:name #$name
+                     #:source #+source
+                     #:make-flags #$make-flags
+                     #:configure-flags #$configure-flags
+                     #:system #$system
+                     #:tests? #$tests?
+                     #:test-target #$test-target
+                     #:build-target #$build-target
+                     #:jar-name #$jar-name
+                     #:main-class #$main-class
+                     #:test-include (list #$@test-include)
+                     #:test-exclude (list #$@test-exclude)
+                     #:source-dir #$source-dir
+                     #:test-dir #$test-dir
+                     #:phases #$phases
+                     #:outputs #$(outputs->gexp outputs)
+                     #:search-paths '#$(map search-path-specification->sexp
+                                            search-paths)
+                     #:inputs #$(input-tuples->gexp inputs)))))
 
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                               ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
-
-  (build-expression->derivation store name builder
-                                #:inputs inputs
-                                #:system system
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build))
+  (mlet %store-monad  ((guile (package->derivation (or guile (default-guile))
+                                                   system #:graft? #f)))
+    (gexp->derivation name builder
+                      #:system system
+                      #:guile-for-build guile)))
 
 (define ant-build-system
   (build-system
diff --git a/guix/build-system/asdf.scm b/guix/build-system/asdf.scm
index 28403a1960..5f01d7ccce 100644
--- a/guix/build-system/asdf.scm
+++ b/guix/build-system/asdf.scm
@@ -1,6 +1,7 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2016, 2017 Andy Patterson <ajpatter@uwaterloo.ca>
 ;;; Copyright © 2019, 2020 Guillaume Le Vaillant <glv@posteo.net>
+;;; Copyright © 2021 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -22,7 +23,8 @@
   #:use-module (guix utils)
   #:use-module (guix memoization)
   #:use-module (guix packages)
-  #:use-module (guix derivations)
+  #:use-module (guix gexp)
+  #:use-module (guix monads)
   #:use-module (guix search-paths)
   #:use-module ((guix build utils)
                 #:select ((package-name->name+version
@@ -92,7 +94,7 @@
          (build asdf-build/source)
          (arguments (strip-keyword-arguments private-keywords arguments)))))
 
-(define* (asdf-build/source store name inputs
+(define* (asdf-build/source name inputs
                             #:key source outputs
                             (phases '(@ (guix build asdf-build-system)
                                         %standard-phases/source))
@@ -102,36 +104,23 @@
                             (imported-modules %asdf-build-system-modules)
                             (modules %asdf-build-modules))
   (define builder
-    `(begin
-       (use-modules ,@modules)
-       (asdf-build/source #:name ,name
-                          #:source ,(match (assoc-ref inputs "source")
-                                      (((? derivation? source))
-                                       (derivation->output-path source))
-                                      ((source) source)
-                                      (source source))
-                          #:system ,system
-                          #:phases ,phases
-                          #:outputs %outputs
-                          #:search-paths ',(map search-path-specification->sexp
-                                                search-paths)
-                          #:inputs %build-inputs)))
-
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
-
-  (build-expression->derivation store name builder
-                                #:inputs inputs
-                                #:system system
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
+          (asdf-build/source #:name #$name
+                             #:source #+source
+                             #:system #$system
+                             #:phases #$phases
+                             #:outputs #$(outputs->gexp outputs)
+                             #:search-paths '#$(map search-path-specification->sexp
+                                                    search-paths)
+                             #:inputs #$(input-tuples->gexp inputs)))))
+
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name builder
+                      #:system system
+                      #:guile-for-build guile)))
 
 (define* (package-with-build-system from-build-system to-build-system
                                     from-prefix to-prefix
@@ -277,19 +266,19 @@ set up using CL source package conventions."
            (arguments (strip-keyword-arguments private-keywords arguments))))))
 
 (define (asdf-build lisp-type)
-  (lambda* (store name inputs
-                  #:key source outputs
-                  (tests? #t)
-                  (asd-files ''())
-                  (asd-systems ''())
-                  (test-asd-file #f)
-                  (phases '(@ (guix build asdf-build-system)
-                              %standard-phases))
-                  (search-paths '())
-                  (system (%current-system))
-                  (guile #f)
-                  (imported-modules %asdf-build-system-modules)
-                  (modules %asdf-build-modules))
+  (lambda* (name inputs
+                 #:key source outputs
+                 (tests? #t)
+                 (asd-files ''())
+                 (asd-systems ''())
+                 (test-asd-file #f)
+                 (phases '(@ (guix build asdf-build-system)
+                             %standard-phases))
+                 (search-paths '())
+                 (system (%current-system))
+                 (guile #f)
+                 (imported-modules %asdf-build-system-modules)
+                 (modules %asdf-build-modules))
 
     ;; FIXME: The definition of 'systems' is pretty hacky.
     ;; Is there a more elegant way to do it?
@@ -300,48 +289,35 @@ set up using CL source package conventions."
               (string-drop
                ;; NAME is the value returned from `package-full-name'.
                (hyphen-separated-name->name+version name)
-               (1+ (string-length lisp-type))))) ; drop the "<lisp>-" prefix.
+               (1+ (string-length lisp-type)))))  ; drop the "<lisp>-" prefix.
           asd-systems))
 
     (define builder
-      `(begin
-         (use-modules ,@modules)
-         (parameterize ((%lisp (string-append
-                                (assoc-ref %build-inputs ,lisp-type)
-                                "/bin/" ,lisp-type))
-                        (%lisp-type ,lisp-type))
-           (asdf-build #:name ,name
-                       #:source ,(match (assoc-ref inputs "source")
-                                   (((? derivation? source))
-                                    (derivation->output-path source))
-                                   ((source) source)
-                                   (source source))
-                       #:asd-files ,asd-files
-                       #:asd-systems ,systems
-                       #:test-asd-file ,test-asd-file
-                       #:system ,system
-                       #:tests? ,tests?
-                       #:phases ,phases
-                       #:outputs %outputs
-                       #:search-paths ',(map search-path-specification->sexp
-                                             search-paths)
-                       #:inputs %build-inputs))))
-
-    (define guile-for-build
-      (match guile
-        ((? package?)
-         (package-derivation store guile system #:graft? #f))
-        (#f
-         (let* ((distro (resolve-interface '(gnu packages commencement)))
-                (guile (module-ref distro 'guile-final)))
-           (package-derivation store guile system #:graft? #f)))))
-
-    (build-expression->derivation store name builder
-                                  #:inputs inputs
-                                  #:system system
-                                  #:modules imported-modules
-                                  #:outputs outputs
-                                  #:guile-for-build guile-for-build)))
+      (with-imported-modules imported-modules
+        #~(begin
+            (use-modules #$@modules)
+            (parameterize ((%lisp (string-append
+                                   (assoc-ref %build-inputs #$lisp-type)
+                                   "/bin/" #$lisp-type))
+                           (%lisp-type #$lisp-type))
+              (asdf-build #:name #$name
+                          #:source #+source
+                          #:asd-files #$asd-files
+                          #:asd-systems #$systems
+                          #:test-asd-file #$test-asd-file
+                          #:system #$system
+                          #:tests? #$tests?
+                          #:phases #$phases
+                          #:outputs #$(outputs->gexp outputs)
+                          #:search-paths '#$(map search-path-specification->sexp
+                                                 search-paths)
+                          #:inputs #$(input-tuples->gexp inputs))))))
+
+    (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                    system #:graft? #f)))
+      (gexp->derivation name builder
+                        #:system system
+                        #:guile-for-build guile))))
 
 (define asdf-build-system/sbcl
   (build-system
diff --git a/guix/build-system/cargo.scm b/guix/build-system/cargo.scm
index 0c76ba9355..d29265de7d 100644
--- a/guix/build-system/cargo.scm
+++ b/guix/build-system/cargo.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2013, 2014, 2015, 2016, 2019 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2013, 2014, 2015, 2016, 2019, 2021 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2013 Andreas Enge <andreas@enge.fr>
 ;;; Copyright © 2013 Nikita Karetnikov <nikita@karetnikov.org>
 ;;; Copyright © 2016 David Craven <david@craven.ch>
@@ -26,7 +26,8 @@
   #:use-module (guix search-paths)
   #:use-module (guix store)
   #:use-module (guix utils)
-  #:use-module (guix derivations)
+  #:use-module (guix gexp)
+  #:use-module (guix monads)
   #:use-module (guix packages)
   #:use-module (guix build-system)
   #:use-module (guix build-system gnu)
@@ -71,8 +72,9 @@ to NAME and VERSION."
     (guix build json)
     ,@%cargo-utils-modules))
 
-(define* (cargo-build store name inputs
+(define* (cargo-build name inputs
                       #:key
+                      source
                       (tests? #t)
                       (test-target #f)
                       (vendor-dir "guix-vendor")
@@ -94,47 +96,37 @@ to NAME and VERSION."
   "Build SOURCE using CARGO, and with INPUTS."
 
   (define builder
-    `(begin
-       (use-modules ,@modules)
-       (cargo-build #:name ,name
-                    #:source ,(match (assoc-ref inputs "source")
-                                (((? derivation? source))
-                                 (derivation->output-path source))
-                                ((source)
-                                 source)
-                                (source
-                                 source))
-                    #:system ,system
-                    #:test-target ,test-target
-                    #:vendor-dir ,vendor-dir
-                    #:cargo-build-flags ,cargo-build-flags
-                    #:cargo-test-flags ,cargo-test-flags
-                    #:cargo-package-flags ,cargo-package-flags
-                    #:features ,features
-                    #:skip-build? ,skip-build?
-                    #:install-source? ,install-source?
-                    #:tests? ,(and tests? (not skip-build?))
-                    #:phases ,phases
-                    #:outputs %outputs
-                    #:search-paths ',(map search-path-specification->sexp
-                                          search-paths)
-                    #:inputs %build-inputs)))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
 
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
+          (cargo-build #:name #$name
+                       #:source #+source
+                       #:system #$system
+                       #:test-target #$test-target
+                       #:vendor-dir #$vendor-dir
+                       #:cargo-build-flags #$cargo-build-flags
+                       #:cargo-test-flags #$cargo-test-flags
+                       #:cargo-package-flags #$cargo-package-flags
+                       #:features #$features
+                       #:skip-build? #$skip-build?
+                       #:install-source? #$install-source?
+                       #:tests? #$(and tests? (not skip-build?))
+                       #:phases #$phases
+                       #:outputs (list #$@(map (lambda (name)
+                                                 #~(cons #$name
+                                                         (ungexp output name)))
+                                               outputs))
+                       #:inputs (map (lambda (tuple)
+                                       (apply cons tuple))
+                                     '#$inputs)
+                       #:search-paths '#$(map search-path-specification->sexp
+                                             search-paths)))))
 
-  (build-expression->derivation store name builder
-                                #:inputs inputs
-                                #:system system
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build))
+  (gexp->derivation name builder
+                    #:system system
+                    #:target #f
+                    #:guile-for-build guile))
 
 (define (package-cargo-inputs p)
   (apply
@@ -253,7 +245,7 @@ any dependent crates. This can be a benefits:
   "Return a bag for NAME."
 
   (define private-keywords
-    '(#:source #:target #:rust #:inputs #:native-inputs #:outputs
+    '(#:target #:rust #:inputs #:native-inputs #:outputs
       #:cargo-inputs #:cargo-development-inputs))
 
   (and (not target) ;; TODO: support cross-compilation
diff --git a/guix/build-system/chicken.scm b/guix/build-system/chicken.scm
index 9abae0431a..0989e7a79f 100644
--- a/guix/build-system/chicken.scm
+++ b/guix/build-system/chicken.scm
@@ -1,5 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2020 raingloom <raingloom@riseup.net>
+;;; Copyright © 2021 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -18,7 +19,9 @@
 
 (define-module (guix build-system chicken)
   #:use-module (guix utils)
-  #:use-module (guix derivations)
+  #:use-module (guix gexp)
+  #:use-module (guix store)
+  #:use-module (guix monads)
   #:use-module (guix search-paths)
   #:use-module (guix build-system)
   #:use-module (guix build-system gnu)
@@ -47,7 +50,7 @@
                 #:rest arguments)
   "Return a bag for NAME."
   (define private-keywords
-    '(#:source #:target #:chicken #:inputs #:native-inputs))
+    '(#:target #:chicken #:inputs #:native-inputs))
 
   ;; TODO: cross-compilation support
   (and (not target)
@@ -69,60 +72,45 @@
          (build chicken-build)
          (arguments (strip-keyword-arguments private-keywords arguments)))))
 
-(define* (chicken-build store name inputs
-                   #:key
-                   (phases '(@ (guix build chicken-build-system)
-                               %standard-phases))
-                   (outputs '("out"))
-                   (search-paths '())
-                   (egg-name "")
-                   (unpack-path "")
-                   (build-flags ''())
-                   (tests? #t)
-                   (system (%current-system))
-                   (guile #f)
-                   (imported-modules %chicken-build-system-modules)
-                   (modules '((guix build chicken-build-system)
-                              (guix build union)
-                              (guix build utils))))
+(define* (chicken-build name inputs
+                        #:key
+                        source
+                        (phases '(@ (guix build chicken-build-system)
+                                    %standard-phases))
+                        (outputs '("out"))
+                        (search-paths '())
+                        (egg-name "")
+                        (unpack-path "")
+                        (build-flags ''())
+                        (tests? #t)
+                        (system (%current-system))
+                        (guile #f)
+                        (imported-modules %chicken-build-system-modules)
+                        (modules '((guix build chicken-build-system)
+                                   (guix build union)
+                                   (guix build utils))))
   (define builder
-    `(begin
-       (use-modules ,@modules)
-       (chicken-build #:name ,name
-                 #:source ,(match (assoc-ref inputs "source")
-                             (((? derivation? source))
-                              (derivation->output-path source))
-                             ((source)
-                              source)
-                             (source
-                              source))
-                 #:system ,system
-                 #:phases ,phases
-                 #:outputs %outputs
-                 #:search-paths ',(map search-path-specification->sexp
-                                       search-paths)
-                 #:egg-name ,egg-name
-                 #:unpack-path ,unpack-path
-                 #:build-flags ,build-flags
-                 #:tests? ,tests?
-                 #:inputs %build-inputs)))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
+          (chicken-build #:name #$name
+                         #:source #+source
+                         #:system #$system
+                         #:phases #$phases
+                         #:outputs #$(outputs->gexp outputs)
+                         #:search-paths '#$(map search-path-specification->sexp
+                                                search-paths)
+                         #:egg-name #$egg-name
+                         #:unpack-path #$unpack-path
+                         #:build-flags #$build-flags
+                         #:tests? #$tests?
+                         #:inputs #$(input-tuples->gexp inputs)))))
 
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system
-                             #:graft? #f)))))
-
-  (build-expression->derivation store name builder
-                                #:inputs inputs
-                                #:system system
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build))
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name builder
+                      #:system system
+                      #:guile-for-build guile)))
 
 (define chicken-build-system
   (build-system
diff --git a/guix/build-system/clojure.scm b/guix/build-system/clojure.scm
index 607f67aaec..e2ad67e3b6 100644
--- a/guix/build-system/clojure.scm
+++ b/guix/build-system/clojure.scm
@@ -1,6 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2018 Alex Vong <alexvong1995@gmail.com>
-;;; Copyright © 2020 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2020, 2021 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -25,7 +25,9 @@
                 #:select (standard-packages)
                 #:prefix gnu:)
 
-  #:use-module (guix derivations)
+  #:use-module (guix gexp)
+  #:use-module (guix store)
+  #:use-module (guix monads)
   #:use-module (guix packages)
   #:use-module ((guix search-paths)
                 #:select
@@ -102,26 +104,9 @@
              (arguments (strip-keyword-arguments private-keywords
                                                  arguments))))))
 
-(define-with-docs source->output-path
-  "Convert source input to output path."
-  (match-lambda
-    (((? derivation? source))
-     (derivation->output-path source))
-    ((source)
-     source)
-    (source
-     source)))
-
-(define-with-docs maybe-guile->guile
-  "Find the right guile."
-  (match-lambda
-    ((and maybe-guile (? package?))
-     maybe-guile)
-    (#f ; default
-     (@* (gnu packages commencement) guile-final))))
-
-(define* (clojure-build store name inputs
+(define* (clojure-build name inputs
                         #:key
+                        source
                         (source-dirs `',%source-dirs)
                         (test-dirs `',%test-dirs)
                         (compile-dir %compile-dir)
@@ -133,7 +118,7 @@
                         (aot-include `',%aot-include)
                         (aot-exclude `',%aot-exclude)
 
-                        doc-dirs ; no sensible default
+                        doc-dirs                  ; no sensible default
                         (doc-regex %doc-regex)
 
                         (tests? %tests?)
@@ -149,48 +134,44 @@
                         (imported-modules %clojure-build-system-modules)
                         (modules %default-modules))
   "Build SOURCE with INPUTS."
-  (let ((builder `(begin
-                    (use-modules ,@modules)
-                    (clojure-build #:name ,name
-                                   #:source ,(source->output-path
-                                              (assoc-ref inputs "source"))
-
-                                   #:source-dirs ,source-dirs
-                                   #:test-dirs ,test-dirs
-                                   #:compile-dir ,compile-dir
-
-                                   #:jar-names ,jar-names
-                                   #:main-class ,main-class
-                                   #:omit-source? ,omit-source?
-
-                                   #:aot-include ,aot-include
-                                   #:aot-exclude ,aot-exclude
-
-                                   #:doc-dirs ,doc-dirs
-                                   #:doc-regex ,doc-regex
-
-                                   #:tests? ,tests?
-                                   #:test-include ,test-include
-                                   #:test-exclude ,test-exclude
-
-                                   #:phases ,phases
-                                   #:outputs %outputs
-                                   #:search-paths ',(map search-path-spec->sexp
-                                                         search-paths)
-                                   #:system ,system
-                                   #:inputs %build-inputs)))
-
-        (guile-for-build (package-derivation store
-                                             (maybe-guile->guile guile)
-                                             system
-                                             #:graft? #f)))
-
-    (build-expression->derivation store name builder
-                                  #:inputs inputs
-                                  #:system system
-                                  #:modules imported-modules
-                                  #:outputs outputs
-                                  #:guile-for-build guile-for-build)))
+  (define builder
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
+
+          (clojure-build #:name #$name
+                         #:source #+source
+
+                         #:source-dirs #$source-dirs
+                         #:test-dirs #$test-dirs
+                         #:compile-dir #$compile-dir
+
+                         #:jar-names #$jar-names
+                         #:main-class #$main-class
+                         #:omit-source? #$omit-source?
+
+                         #:aot-include #$aot-include
+                         #:aot-exclude #$aot-exclude
+
+                         #:doc-dirs #$doc-dirs
+                         #:doc-regex #$doc-regex
+
+                         #:tests? #$tests?
+                         #:test-include #$test-include
+                         #:test-exclude #$test-exclude
+
+                         #:phases #$phases
+                         #:outputs #$(outputs->gexp outputs)
+                         #:search-paths '#$(map search-path-spec->sexp
+                                                search-paths)
+                         #:system #$system
+                         #:inputs #$(input-tuples->gexp inputs)))))
+
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name builder
+                      #:system system
+                      #:guile-for-build guile)))
 
 (define clojure-build-system
   (build-system
diff --git a/guix/build-system/cmake.scm b/guix/build-system/cmake.scm
index f590b6ea42..f9ac2befc9 100644
--- a/guix/build-system/cmake.scm
+++ b/guix/build-system/cmake.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2013, 2014, 2015, 2020 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2013, 2014, 2015, 2020, 2021 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2013 Cyril Roelandt <tipecaml@gmail.com>
 ;;; Copyright © 2017 Ricardo Wurmus <rekado@elephly.net>
 ;;; Copyright © 2020 Efraim Flashner <efraim@flashner.co.il>
@@ -21,7 +21,9 @@
 
 (define-module (guix build-system cmake)
   #:use-module (guix store)
+  #:use-module (guix gexp)
   #:use-module (guix utils)
+  #:use-module (guix monads)
   #:use-module (guix derivations)
   #:use-module (guix search-paths)
   #:use-module (guix build-system)
@@ -61,7 +63,7 @@
                 #:rest arguments)
   "Return a bag for NAME."
   (define private-keywords
-    `(#:source #:cmake #:inputs #:native-inputs #:outputs
+    `(#:cmake #:inputs #:native-inputs
       ,@(if target '() '(#:target))))
 
   (bag
@@ -95,8 +97,8 @@
     (build (if target cmake-cross-build cmake-build))
     (arguments (strip-keyword-arguments private-keywords arguments))))
 
-(define* (cmake-build store name inputs
-                      #:key (guile #f)
+(define* (cmake-build name inputs
+                      #:key guile source
                       (outputs '("out")) (configure-flags ''())
                       (search-paths '())
                       (make-flags ''())
@@ -120,62 +122,51 @@
                                  (guix build utils))))
   "Build SOURCE using CMAKE, and with INPUTS. This assumes that SOURCE
 provides a 'CMakeLists.txt' file as its build system."
-  (define builder
-    `(begin
-       (use-modules ,@modules)
-       (cmake-build #:source ,(match (assoc-ref inputs "source")
-                                (((? derivation? source))
-                                 (derivation->output-path source))
-                                ((source)
-                                 source)
-                                (source
-                                 source))
-                    #:system ,system
-                    #:outputs %outputs
-                    #:inputs %build-inputs
-                    #:search-paths ',(map search-path-specification->sexp
-                                          search-paths)
-                    #:phases ,phases
-                    #:configure-flags ,configure-flags
-                    #:make-flags ,make-flags
-                    #:out-of-source? ,out-of-source?
-                    #:build-type ,build-type
-                    #:tests? ,tests?
-                    #:test-target ,test-target
-                    #:parallel-build? ,parallel-build?
-                    #:parallel-tests? ,parallel-tests?
-                    #:validate-runpath? ,validate-runpath?
-                    #:patch-shebangs? ,patch-shebangs?
-                    #:strip-binaries? ,strip-binaries?
-                    #:strip-flags ,strip-flags
-                    #:strip-directories ,strip-directories)))
+  (define build
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
 
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
+          #$(with-build-variables inputs outputs
+              #~(cmake-build #:source #+source
+                             #:system #$system
+                             #:outputs %outputs
+                             #:inputs %build-inputs
+                             #:search-paths '#$(map search-path-specification->sexp
+                                                    search-paths)
+                             #:phases #$phases
+                             #:configure-flags #$configure-flags
+                             #:make-flags #$make-flags
+                             #:out-of-source? #$out-of-source?
+                             #:build-type #$build-type
+                             #:tests? #$tests?
+                             #:test-target #$test-target
+                             #:parallel-build? #$parallel-build?
+                             #:parallel-tests? #$parallel-tests?
+                             #:validate-runpath? #$validate-runpath?
+                             #:patch-shebangs? #$patch-shebangs?
+                             #:strip-binaries? #$strip-binaries?
+                             #:strip-flags #$strip-flags
+                             #:strip-directories #$strip-directories)))))
 
-  (build-expression->derivation store name builder
-                                #:system system
-                                #:inputs inputs
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:substitutable? substitutable?
-                                #:guile-for-build guile-for-build))
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name build
+                      #:system system
+                      #:target #f
+                      #:substitutable? substitutable?
+                      #:guile-for-build guile)))
 
 
 ;;;
 ;;; Cross-compilation.
 ;;;
 
-(define* (cmake-cross-build store name
+(define* (cmake-cross-build name
                             #:key
-                            target native-drvs target-drvs
-                            (guile #f)
+                            target
+                            build-inputs target-inputs host-inputs
+                            source guile
                             (outputs '("out"))
                             (configure-flags ''())
                             (search-paths '())
@@ -205,78 +196,60 @@ provides a 'CMakeLists.txt' file as its build system."
 with INPUTS.  This assumes that SOURCE provides a 'CMakeLists.txt' file as its
 build system."
   (define builder
-    `(begin
-       (use-modules ,@modules)
-       (let ()
-         (define %build-host-inputs
-           ',(map (match-lambda
-                    ((name (? derivation? drv) sub ...)
-                     `(,name . ,(apply derivation->output-path drv sub)))
-                    ((name path)
-                     `(,name . ,path)))
-                  native-drvs))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@(sexp->gexp modules))
+
+          (define %build-host-inputs
+            (map (lambda (tuple)
+                   (apply cons tuple))
+                 '#+(append build-inputs target-inputs)))
 
-         (define %build-target-inputs
-           ',(map (match-lambda
-                    ((name (? derivation? drv) sub ...)
-                     `(,name . ,(apply derivation->output-path drv sub)))
-                    ((name (? package? pkg) sub ...)
-                     (let ((drv (package-cross-derivation store pkg
-                                                          target system)))
-                       `(,name . ,(apply derivation->output-path drv sub))))
-                    ((name path)
-                     `(,name . ,path)))
-                  target-drvs))
+          (define %build-target-inputs
+            (map (lambda (tuple)
+                   (apply cons tuple))
+                 '#$host-inputs))
 
-         (cmake-build #:source ,(match (assoc-ref native-drvs "source")
-                                  (((? derivation? source))
-                                   (derivation->output-path source))
-                                  ((source)
-                                   source)
-                                  (source
-                                   source))
-                      #:system ,system
-                      #:build ,build
-                      #:target ,target
-                      #:outputs %outputs
-                      #:inputs %build-target-inputs
-                      #:native-inputs %build-host-inputs
-                      #:search-paths ',(map search-path-specification->sexp
-                                            search-paths)
-                      #:native-search-paths ',(map
-                                               search-path-specification->sexp
-                                               native-search-paths)
-                      #:phases ,phases
-                      #:configure-flags ,configure-flags
-                      #:make-flags ,make-flags
-                      #:out-of-source? ,out-of-source?
-                      #:build-type ,build-type
-                      #:tests? ,tests?
-                      #:test-target ,test-target
-                      #:parallel-build? ,parallel-build?
-                      #:parallel-tests? ,parallel-tests?
-                      #:validate-runpath? ,validate-runpath?
-                      #:patch-shebangs? ,patch-shebangs?
-                      #:strip-binaries? ,strip-binaries?
-                      #:strip-flags ,strip-flags
-                      #:strip-directories ,strip-directories))))
+          (define %outputs
+            (list #$@(map (lambda (name)
+                            #~(cons #$name
+                                    (ungexp output name)))
+                          outputs)))
 
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                               ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
+          (cmake-build #:source #+source
+                       #:system #$system
+                       #:build #$build
+                       #:target #$target
+                       #:outputs %outputs
+                       #:inputs %build-target-inputs
+                       #:native-inputs %build-host-inputs
+                       #:search-paths '#$(map search-path-specification->sexp
+                                              search-paths)
+                       #:native-search-paths '#$(map
+                                                 search-path-specification->sexp
+                                                 native-search-paths)
+                       #:phases #$phases
+                       #:configure-flags #$configure-flags
+                       #:make-flags #$make-flags
+                       #:out-of-source? #$out-of-source?
+                       #:build-type #$build-type
+                       #:tests? #$tests?
+                       #:test-target #$test-target
+                       #:parallel-build? #$parallel-build?
+                       #:parallel-tests? #$parallel-tests?
+                       #:validate-runpath? #$validate-runpath?
+                       #:patch-shebangs? #$patch-shebangs?
+                       #:strip-binaries? #$strip-binaries?
+                       #:strip-flags #$strip-flags
+                       #:strip-directories #$strip-directories))))
 
-  (build-expression->derivation store name builder
-                                #:system system
-                                #:inputs (append native-drvs target-drvs)
-                                #:outputs outputs
-                                #:modules imported-modules
-                                #:substitutable? substitutable?
-                                #:guile-for-build guile-for-build))
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name builder
+                      #:system system
+                      #:target target
+                      #:substitutable? substitutable?
+                      #:guile-for-build guile)))
 
 (define cmake-build-system
   (build-system
diff --git a/guix/build-system/copy.scm b/guix/build-system/copy.scm
index d1bf8fb654..8dea0b4c6b 100644
--- a/guix/build-system/copy.scm
+++ b/guix/build-system/copy.scm
@@ -1,6 +1,7 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2019 Julien Lepiller <julien@lepiller.eu>
 ;;; Copyright © 2020 Pierre Neidhardt <mail@ambrevar.xyz>
+;;; Copyright © 2021 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -20,7 +21,8 @@
 (define-module (guix build-system copy)
   #:use-module (guix store)
   #:use-module (guix utils)
-  #:use-module (guix derivations)
+  #:use-module (guix gexp)
+  #:use-module (guix monads)
   #:use-module (guix search-paths)
   #:use-module (guix build-system)
   #:use-module (guix build-system gnu)
@@ -59,7 +61,7 @@
                 #:rest arguments)
   "Return a bag for NAME from the given arguments."
   (define private-keywords
-    '(#:source #:target #:inputs #:native-inputs))
+    '(#:target #:inputs #:native-inputs))
 
   (bag
     (name name)
@@ -75,8 +77,9 @@
     (build copy-build)
     (arguments (strip-keyword-arguments private-keywords arguments))))
 
-(define* (copy-build store name inputs
-                     #:key (guile #f)
+(define* (copy-build name inputs
+                     #:key
+                     guile source
                      (outputs '("out"))
                      (install-plan ''(("." "./")))
                      (search-paths '())
@@ -90,49 +93,38 @@
                      (phases '(@ (guix build copy-build-system)
                                  %standard-phases))
                      (system (%current-system))
+                     (target #f)
                      (imported-modules %copy-build-system-modules)
                      (modules '((guix build copy-build-system)
                                 (guix build utils))))
   "Build SOURCE using INSTALL-PLAN, and with INPUTS."
   (define builder
-    `(begin
-       (use-modules ,@modules)
-       (copy-build #:source ,(match (assoc-ref inputs "source")
-                               (((?  derivation? source))
-                                (derivation->output-path source))
-                               ((source)
-                                source)
-                               (source
-                                source))
-                   #:system ,system
-                   #:outputs %outputs
-                   #:inputs %build-inputs
-                   #:install-plan ,install-plan
-                   #:search-paths ',(map search-path-specification->sexp
-                                         search-paths)
-                   #:phases ,phases
-                   #:out-of-source? ,out-of-source?
-                   #:validate-runpath? ,validate-runpath?
-                   #:patch-shebangs? ,patch-shebangs?
-                   #:strip-binaries? ,strip-binaries?
-                   #:strip-flags ,strip-flags
-                   #:strip-directories ,strip-directories)))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
 
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
+          #$(with-build-variables inputs outputs
+              #~(copy-build #:source #+source
+                            #:system #$system
+                            #:outputs %outputs
+                            #:inputs %build-inputs
+                            #:install-plan #$install-plan
+                            #:search-paths '#$(map search-path-specification->sexp
+                                                   search-paths)
+                            #:phases #$phases
+                            #:out-of-source? #$out-of-source?
+                            #:validate-runpath? #$validate-runpath?
+                            #:patch-shebangs? #$patch-shebangs?
+                            #:strip-binaries? #$strip-binaries?
+                            #:strip-flags #$strip-flags
+                            #:strip-directories #$strip-directories)))))
 
-  (build-expression->derivation store name builder
-                                #:system system
-                                #:inputs inputs
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build))
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name builder
+                      #:system system
+                      #:target #f
+                      #:guile-for-build guile)))
 
 (define copy-build-system
   (build-system
diff --git a/guix/build-system/dub.scm b/guix/build-system/dub.scm
index 5a31a2f51a..58a72fe828 100644
--- a/guix/build-system/dub.scm
+++ b/guix/build-system/dub.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2013, 2014, 2015, 2016 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2013, 2014, 2015, 2016, 2021 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2013 Andreas Enge <andreas@enge.fr>
 ;;; Copyright © 2013 Nikita Karetnikov <nikita@karetnikov.org>
 ;;; Copyright © 2016 David Craven <david@craven.ch>
@@ -24,7 +24,8 @@
   #:use-module (guix search-paths)
   #:use-module (guix store)
   #:use-module (guix utils)
-  #:use-module (guix derivations)
+  #:use-module (guix gexp)
+  #:use-module (guix monads)
   #:use-module (guix packages)
   #:use-module (guix build-system)
   #:use-module (guix build-system gnu)
@@ -56,57 +57,43 @@
     (guix build syscalls)
     ,@%gnu-build-system-modules))
 
-(define* (dub-build store name inputs
-                      #:key
-                      (tests? #t)
-                      (test-target #f)
-                      (dub-build-flags ''())
-                      (phases '(@ (guix build dub-build-system)
-                                  %standard-phases))
-                      (outputs '("out"))
-                      (search-paths '())
-                      (system (%current-system))
-                      (guile #f)
-                      (imported-modules %dub-build-system-modules)
-                      (modules '((guix build dub-build-system)
-                                 (guix build utils))))
+(define* (dub-build name inputs
+                    #:key
+                    source
+                    (tests? #t)
+                    (test-target #f)
+                    (dub-build-flags ''())
+                    (phases '(@ (guix build dub-build-system)
+                                %standard-phases))
+                    (outputs '("out"))
+                    (search-paths '())
+                    (system (%current-system))
+                    (guile #f)
+                    (imported-modules %dub-build-system-modules)
+                    (modules '((guix build dub-build-system)
+                               (guix build utils))))
   "Build SOURCE using DUB, and with INPUTS."
   (define builder
-    `(begin
-       (use-modules ,@modules)
-       (dub-build #:name ,name
-                    #:source ,(match (assoc-ref inputs "source")
-                                (((? derivation? source))
-                                 (derivation->output-path source))
-                                ((source)
-                                 source)
-                                (source
-                                 source))
-                    #:system ,system
-                    #:test-target ,test-target
-                    #:dub-build-flags ,dub-build-flags
-                    #:tests? ,tests?
-                    #:phases ,phases
-                    #:outputs %outputs
-                    #:search-paths ',(map search-path-specification->sexp
-                                          search-paths)
-                    #:inputs %build-inputs)))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
+          (dub-build #:name #$name
+                     #:source #+source
+                     #:system #$system
+                     #:test-target #$test-target
+                     #:dub-build-flags #$dub-build-flags
+                     #:tests? #$tests?
+                     #:phases #$phases
+                     #:outputs #$(outputs->gexp outputs)
+                     #:search-paths '#$(map search-path-specification->sexp
+                                            search-paths)
+                     #:inputs #$(input-tuples->gexp inputs)))))
 
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
-
-  (build-expression->derivation store name builder
-                                #:inputs inputs
-                                #:system system
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build))
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name builder
+                      #:system system
+                      #:guile-for-build guile)))
 
 (define* (lower name
                 #:key source inputs native-inputs outputs system target
@@ -118,7 +105,7 @@
   "Return a bag for NAME."
 
   (define private-keywords
-    '(#:source #:target #:ldc #:dub #:pkg-config #:inputs #:native-inputs #:outputs))
+    '(#:target #:ldc #:dub #:pkg-config #:inputs #:native-inputs #:outputs))
 
   (and (not target) ;; TODO: support cross-compilation
        (bag
diff --git a/guix/build-system/dune.scm b/guix/build-system/dune.scm
index 6a2f3d16de..8c33e096f5 100644
--- a/guix/build-system/dune.scm
+++ b/guix/build-system/dune.scm
@@ -1,6 +1,7 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2016, 2017, 2018 Julien Lepiller <julien@lepiller.eu>
 ;;; Copyright © 2017 Ben Woodcroft <donttrustben@gmail.com>
+;;; Copyright © 2021 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -20,7 +21,7 @@
 (define-module (guix build-system dune)
   #:use-module (guix store)
   #:use-module (guix utils)
-  #:use-module (guix derivations)
+  #:use-module (guix gexp)
   #:use-module (guix search-paths)
   #:use-module (guix build-system)
   #:use-module ((guix build-system gnu) #:prefix gnu:)
@@ -60,7 +61,7 @@
                 #:rest arguments)
   "Return a bag for NAME."
   (define private-keywords
-    '(#:source #:target #:dune #:findlib #:ocaml #:inputs #:native-inputs))
+    '(#:target #:dune #:findlib #:ocaml #:inputs #:native-inputs))
 
   (and (not target)                               ;XXX: no cross-compilation
        (let ((base (ocaml:lower name
@@ -80,8 +81,9 @@
            (build dune-build)
            (arguments (strip-keyword-arguments private-keywords arguments))))))
 
-(define* (dune-build store name inputs
-                     #:key (guile #f)
+(define* (dune-build name inputs
+                     #:key
+                     guile source
                      (outputs '("out"))
                      (search-paths '())
                      (build-flags ''())
@@ -107,50 +109,39 @@
   "Build SOURCE using OCAML, and with INPUTS. This assumes that SOURCE
 provides a 'setup.ml' file as its build system."
   (define builder
-    `(begin
-       (use-modules ,@modules)
-       (dune-build #:source ,(match (assoc-ref inputs "source")
-                               (((? derivation? source))
-                                (derivation->output-path source))
-                               ((source)
-                                source)
-                               (source
-                                source))
-                   #:system ,system
-                   #:outputs %outputs
-                   #:inputs %build-inputs
-                   #:search-paths ',(map search-path-specification->sexp
-                                         search-paths)
-                   #:phases ,phases
-                   #:test-flags ,test-flags
-                   #:build-flags ,build-flags
-                   #:out-of-source? ,out-of-source?
-                   #:jbuild? ,jbuild?
-                   #:package ,package
-                   #:tests? ,tests?
-                   #:test-target ,test-target
-                   #:install-target ,install-target
-                   #:validate-runpath? ,validate-runpath?
-                   #:patch-shebangs? ,patch-shebangs?
-                   #:strip-binaries? ,strip-binaries?
-                   #:strip-flags ,strip-flags
-                   #:strip-directories ,strip-directories)))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
+          (dune-build #:source #$source
+                      #:system #$system
+                      #:outputs (list #$@(map (lambda (name)
+                                                #~(cons #$name
+                                                        (ungexp output name)))
+                                              outputs))
+                      #:inputs (map (lambda (tuple)
+                                      (apply cons tuple))
+                                    '#$inputs)
+                      #:search-paths '#$(map search-path-specification->sexp
+                                             search-paths)
+                      #:phases #$phases
+                      #:test-flags #$test-flags
+                      #:build-flags #$build-flags
+                      #:out-of-source? #$out-of-source?
+                      #:jbuild? #$jbuild?
+                      #:package #$package
+                      #:tests? #$tests?
+                      #:test-target #$test-target
+                      #:install-target #$install-target
+                      #:validate-runpath? #$validate-runpath?
+                      #:patch-shebangs? #$patch-shebangs?
+                      #:strip-binaries? #$strip-binaries?
+                      #:strip-flags #$strip-flags
+                      #:strip-directories #$strip-directories))))
 
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
-
-  (build-expression->derivation store name builder
-                                #:system system
-                                #:inputs inputs
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build))
+  (gexp->derivation name builder
+                    #:system system
+                    #:target #f
+                    #:guile-for-build guile))
 
 (define dune-build-system
   (build-system
diff --git a/guix/build-system/emacs.scm b/guix/build-system/emacs.scm
index ac05ff420e..0a8f828b3d 100644
--- a/guix/build-system/emacs.scm
+++ b/guix/build-system/emacs.scm
@@ -23,7 +23,8 @@
   #:use-module (guix store)
   #:use-module (guix utils)
   #:use-module (guix packages)
-  #:use-module (guix derivations)
+  #:use-module (guix gexp)
+  #:use-module (guix monads)
   #:use-module (guix search-paths)
   #:use-module (guix build-system)
   #:use-module (guix build-system gnu)
@@ -81,7 +82,7 @@
          (build emacs-build)
          (arguments (strip-keyword-arguments private-keywords arguments)))))
 
-(define* (emacs-build store name inputs
+(define* (emacs-build name inputs
                       #:key source
                       (tests? #f)
                       (parallel-tests? #t)
@@ -100,43 +101,28 @@
                                  (guix build emacs-utils))))
   "Build SOURCE using EMACS, and with INPUTS."
   (define builder
-    `(begin
-       (use-modules ,@modules)
-       (emacs-build #:name ,name
-                    #:source ,(match (assoc-ref inputs "source")
-                                (((? derivation? source))
-                                 (derivation->output-path source))
-                                ((source)
-                                 source)
-                                (source
-                                 source))
-                    #:system ,system
-                    #:test-command ,test-command
-                    #:tests? ,tests?
-                    #:parallel-tests? ,parallel-tests?
-                    #:phases ,phases
-                    #:outputs %outputs
-                    #:include ,include
-                    #:exclude ,exclude
-                    #:search-paths ',(map search-path-specification->sexp
-                                          search-paths)
-                    #:inputs %build-inputs)))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
+          (emacs-build #:name #$name
+                       #:source #+source
+                       #:system #$system
+                       #:test-command #$test-command
+                       #:tests? #$tests?
+                       #:parallel-tests? #$parallel-tests?
+                       #:phases #$phases
+                       #:outputs #$(outputs->gexp outputs)
+                       #:include #$include
+                       #:exclude #$exclude
+                       #:search-paths '#$(map search-path-specification->sexp
+                                              search-paths)
+                       #:inputs #$(input-tuples->gexp inputs)))))
 
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
-
-  (build-expression->derivation store name builder
-                                #:inputs inputs
-                                #:system system
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build))
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name builder
+                      #:system system
+                      #:guile-for-build guile)))
 
 (define emacs-build-system
   (build-system
diff --git a/guix/build-system/font.scm b/guix/build-system/font.scm
index d40a4985f8..e7160ff426 100644
--- a/guix/build-system/font.scm
+++ b/guix/build-system/font.scm
@@ -17,6 +17,9 @@
 ;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
 
 (define-module (guix build-system font)
+  #:use-module (guix gexp)
+  #:use-module (guix store)
+  #:use-module (guix monads)
   #:use-module (guix utils)
   #:use-module (guix packages)
   #:use-module (guix derivations)
@@ -69,7 +72,7 @@
     (build font-build)
     (arguments (strip-keyword-arguments private-keywords arguments))))
 
-(define* (font-build store name inputs
+(define* (font-build name inputs
                      #:key source
                      (tests? #t)
                      (test-target "test")
@@ -85,41 +88,29 @@
                                 (guix build utils))))
   "Build SOURCE with INPUTS."
   (define builder
-    `(begin
-       (use-modules ,@modules)
-       (font-build #:name ,name
-                   #:source ,(match (assoc-ref inputs "source")
-                               (((? derivation? source))
-                                (derivation->output-path source))
-                               ((source)
-                                source)
-                               (source
-                                source))
-                   #:configure-flags ,configure-flags
-                   #:system ,system
-                   #:test-target ,test-target
-                   #:tests? ,tests?
-                   #:phases ,phases
-                   #:outputs %outputs
-                   #:search-paths ',(map search-path-specification->sexp
-                                         search-paths)
-                   #:inputs %build-inputs)))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
 
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
+          #$(with-build-variables inputs outputs
+              #~(font-build #:name #$name
+                            #:source #+source
+                            #:configure-flags #$configure-flags
+                            #:system #$system
+                            #:test-target #$test-target
+                            #:tests? #$tests?
+                            #:phases #$phases
+                            #:outputs %outputs
+                            #:search-paths '#$(map search-path-specification->sexp
+                                                   search-paths)
+                            #:inputs %build-inputs)))))
 
-  (build-expression->derivation store name builder
-                                #:inputs inputs
-                                #:system system
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build))
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name builder
+                      #:system system
+                      #:target #f
+                      #:guile-for-build guile)))
 
 (define font-build-system
   (build-system
diff --git a/guix/build-system/glib-or-gtk.scm b/guix/build-system/glib-or-gtk.scm
index fb1f8fb930..6c09b5a3b7 100644
--- a/guix/build-system/glib-or-gtk.scm
+++ b/guix/build-system/glib-or-gtk.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2013, 2014, 2015, 2019, 2020 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2013, 2014, 2015, 2019, 2020, 2021 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2013 Cyril Roelandt <tipecaml@gmail.com>
 ;;; Copyright © 2014 Federico Beffa <beffa@fbengineering.ch>
 ;;;
@@ -21,6 +21,8 @@
 (define-module (guix build-system glib-or-gtk)
   #:use-module (guix store)
   #:use-module (guix utils)
+  #:use-module (guix gexp)
+  #:use-module (guix monads)
   #:use-module (guix derivations)
   #:use-module (guix search-paths)
   #:use-module (guix build-system)
@@ -85,7 +87,7 @@
                 #:rest arguments)
   "Return a bag for NAME."
   (define private-keywords
-    '(#:source #:target #:glib #:inputs #:native-inputs
+    '(#:target #:glib #:inputs #:native-inputs
       #:outputs #:implicit-inputs?))
 
   (and (not target)                               ;XXX: no cross-compilation
@@ -105,8 +107,8 @@
          (build glib-or-gtk-build)
          (arguments (strip-keyword-arguments private-keywords arguments)))))
 
-(define* (glib-or-gtk-build store name inputs
-                            #:key (guile #f)
+(define* (glib-or-gtk-build name inputs
+                            #:key guile source
                             (outputs '("out"))
                             (search-paths '())
                             (configure-flags ''())
@@ -132,70 +134,43 @@
                             allowed-references
                             disallowed-references)
   "Build SOURCE with INPUTS.  See GNU-BUILD for more details."
-  (define canonicalize-reference
-    (match-lambda
-     ((? package? p)
-      (derivation->output-path (package-derivation store p system)))
-     (((? package? p) output)
-      (derivation->output-path (package-derivation store p system)
-                               output))
-     ((? string? output)
-      output)))
+  (define build
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
 
-  (define builder
-    `(begin
-       (use-modules ,@modules)
-       (glib-or-gtk-build #:source ,(match (assoc-ref inputs "source")
-                                      (((? derivation? source))
-                                       (derivation->output-path source))
-                                      ((source)
-                                       source)
-                                      (source
-                                       source))
-                          #:system ,system
-                          #:outputs %outputs
-                          #:inputs %build-inputs
-                          #:search-paths ',(map search-path-specification->sexp
-                                                search-paths)
-                          #:phases ,phases
-                          #:glib-or-gtk-wrap-excluded-outputs
-                           ,glib-or-gtk-wrap-excluded-outputs
-                          #:configure-flags ,configure-flags
-                          #:make-flags ,make-flags
-                          #:out-of-source? ,out-of-source?
-                          #:tests? ,tests?
-                          #:test-target ,test-target
-                          #:parallel-build? ,parallel-build?
-                          #:parallel-tests? ,parallel-tests?
-                          #:validate-runpath? ,validate-runpath?
-                          #:patch-shebangs? ,patch-shebangs?
-                          #:strip-binaries? ,strip-binaries?
-                          #:strip-flags ,strip-flags
-                          #:strip-directories ,strip-directories)))
+          #$(with-build-variables inputs outputs
+              #~(glib-or-gtk-build #:source #+source
+                                   #:system #$system
+                                   #:outputs %outputs
+                                   #:inputs %build-inputs
+                                   #:search-paths '#$(map search-path-specification->sexp
+                                                          search-paths)
+                                   #:phases #$phases
+                                   #:glib-or-gtk-wrap-excluded-outputs
+                                   #$glib-or-gtk-wrap-excluded-outputs
+                                   #:configure-flags #$configure-flags
+                                   #:make-flags #$make-flags
+                                   #:out-of-source? #$out-of-source?
+                                   #:tests? #$tests?
+                                   #:test-target #$test-target
+                                   #:parallel-build? #$parallel-build?
+                                   #:parallel-tests? #$parallel-tests?
+                                   #:validate-runpath? #$validate-runpath?
+                                   #:patch-shebangs? #$patch-shebangs?
+                                   #:strip-binaries? #$strip-binaries?
+                                   #:strip-flags #$strip-flags
+                                   #:strip-directories #$strip-directories)))))
 
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
 
-  (build-expression->derivation store name builder
-                                #:system system
-                                #:inputs inputs
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:allowed-references
-                                (and allowed-references
-                                     (map canonicalize-reference
-                                          allowed-references))
-                                #:disallowed-references
-                                (and disallowed-references
-                                     (map canonicalize-reference
-                                          disallowed-references))
-                                #:guile-for-build guile-for-build))
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name build
+                      #:system system
+                      #:target #f
+                      #:allowed-references allowed-references
+                      #:disallowed-references disallowed-references
+                      #:guile-for-build guile)))
 
 (define glib-or-gtk-build-system
   (build-system
diff --git a/guix/build-system/gnu.scm b/guix/build-system/gnu.scm
index fc045f10c9..613deb7bb0 100644
--- a/guix/build-system/gnu.scm
+++ b/guix/build-system/gnu.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2012, 2013, 2014, 2015, 2016, 2017, 2019, 2020, 2021 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -20,6 +20,8 @@
   #:use-module (guix store)
   #:use-module (guix utils)
   #:use-module (guix memoization)
+  #:use-module (guix gexp)
+  #:use-module (guix monads)
   #:use-module (guix derivations)
   #:use-module (guix search-paths)
   #:use-module (guix build-system)
@@ -281,7 +283,7 @@ standard packages used as implicit inputs of the GNU build system."
                 #:rest arguments)
   "Return a bag for NAME from the given arguments."
   (define private-keywords
-    `(#:source #:inputs #:native-inputs #:outputs
+    `(#:inputs #:native-inputs #:outputs
       #:implicit-inputs? #:implicit-cross-inputs?
       ,@(if target '() '(#:target))))
 
@@ -328,8 +330,9 @@ standard packages used as implicit inputs of the GNU build system."
   ;; Typical names of Autotools "bootstrap" scripts.
   '("bootstrap" "bootstrap.sh" "autogen.sh"))
 
-(define* (gnu-build store name input-drvs
-                    #:key (guile #f)
+(define* (gnu-build name inputs
+                    #:key
+                    guile source
                     (outputs '("out"))
                     (search-paths '())
                     (bootstrap-scripts (list 'quote %bootstrap-scripts))
@@ -374,80 +377,48 @@ SUBSTITUTABLE? determines whether users may be able to use substitutes of the
 returned derivations, or whether they should always build it locally.
 
 ALLOWED-REFERENCES can be either #f, or a list of packages that the outputs
-are allowed to refer to.  Likewise for DISALLOWED-REFERENCES, which lists
-packages that must not be referenced."
-  (define canonicalize-reference
-    (match-lambda
-     ((? package? p)
-      (derivation->output-path (package-derivation store p system
-                                                   #:graft? #f)))
-     (((? package? p) output)
-      (derivation->output-path (package-derivation store p system
-                                                   #:graft? #f)
-                               output))
-     ((? string? output)
-      output)))
-
+are allowed to refer to."
   (define builder
-    `(begin
-       (use-modules ,@modules)
-       (gnu-build #:source ,(match (assoc-ref input-drvs "source")
-                              (((? derivation? source))
-                               (derivation->output-path source))
-                              ((source)
-                               source)
-                              (source
-                               source))
-                  #:system ,system
-                  #:build ,build
-                  #:outputs %outputs
-                  #:inputs %build-inputs
-                  #:search-paths ',(map search-path-specification->sexp
-                                        search-paths)
-                  #:phases ,phases
-                  #:locale ,locale
-                  #:bootstrap-scripts ,bootstrap-scripts
-                  #:configure-flags ,configure-flags
-                  #:make-flags ,make-flags
-                  #:out-of-source? ,out-of-source?
-                  #:tests? ,tests?
-                  #:test-target ,test-target
-                  #:parallel-build? ,parallel-build?
-                  #:parallel-tests? ,parallel-tests?
-                  #:patch-shebangs? ,patch-shebangs?
-                  #:strip-binaries? ,strip-binaries?
-                  #:validate-runpath? ,validate-runpath?
-                  #:make-dynamic-linker-cache? ,make-dynamic-linker-cache?
-                  #:license-file-regexp ,license-file-regexp
-                  #:strip-flags ,strip-flags
-                  #:strip-directories ,strip-directories)))
-
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system
-                             #:graft? #f)))))
-
-  (build-expression->derivation store name builder
-                                #:system system
-                                #:inputs input-drvs
-                                #:outputs outputs
-                                #:modules imported-modules
-                                #:substitutable? substitutable?
-
-                                #:allowed-references
-                                (and allowed-references
-                                     (map canonicalize-reference
-                                          allowed-references))
-                                #:disallowed-references
-                                (and disallowed-references
-                                     (map canonicalize-reference
-                                          disallowed-references))
-                                #:guile-for-build guile-for-build))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
+
+          #$(with-build-variables inputs outputs
+              #~(gnu-build #:source #+source
+                           #:system #$system
+                           #:build #$build
+                           #:outputs %outputs
+                           #:inputs %build-inputs
+                           #:search-paths '#$(map search-path-specification->sexp
+                                                  search-paths)
+                           #:phases #$phases
+                           #:locale #$locale
+                           #:bootstrap-scripts #$bootstrap-scripts
+                           #:configure-flags #$configure-flags
+                           #:make-flags #$make-flags
+                           #:out-of-source? #$out-of-source?
+                           #:tests? #$tests?
+                           #:test-target #$test-target
+                           #:parallel-build? #$parallel-build?
+                           #:parallel-tests? #$parallel-tests?
+                           #:patch-shebangs? #$patch-shebangs?
+                           #:license-file-regexp #$license-file-regexp
+                           #:strip-binaries? #$strip-binaries?
+                           #:validate-runpath? #$validate-runpath?
+                           #:make-dynamic-linker-cache? #$make-dynamic-linker-cache?
+                           #:license-file-regexp #$license-file-regexp
+                           #:strip-flags #$strip-flags
+                           #:strip-directories #$strip-directories)))))
+
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name builder
+                      #:system system
+                      #:target #f
+                      #:substitutable? substitutable?
+                      #:allowed-references allowed-references
+                      #:disallowed-references disallowed-references
+                      #:guile-for-build guile)))
 
 
 ;;;
@@ -483,11 +454,11 @@ is one of `host' or `target'."
                    `(("cross-libc:static" ,libc "static"))
                    '()))))))))
 
-(define* (gnu-cross-build store name
+(define* (gnu-cross-build name
                           #:key
-                          target native-drvs target-drvs
-                          (guile #f)
-                          source
+                          target
+                          build-inputs target-inputs host-inputs
+                          guile source
                           (outputs '("out"))
                           (search-paths '())
                           (native-search-paths '())
@@ -525,104 +496,67 @@ is one of `host' or `target'."
   "Cross-build NAME for TARGET, where TARGET is a GNU triplet.  INPUTS are
 cross-built inputs, and NATIVE-INPUTS are inputs that run on the build
 platform."
-  (define canonicalize-reference
-    (match-lambda
-     ((? package? p)
-      (derivation->output-path (package-cross-derivation store p
-                                                         target system)))
-     (((? package? p) output)
-      (derivation->output-path (package-cross-derivation store p
-                                                         target system)
-                               output))
-     ((? string? output)
-      output)))
-
   (define builder
-    `(begin
-       (use-modules ,@modules)
-
-       (let ()
-         (define %build-host-inputs
-           ',(map (match-lambda
-                   ((name (? derivation? drv) sub ...)
-                    `(,name . ,(apply derivation->output-path drv sub)))
-                   ((name path)
-                    `(,name . ,path)))
-                  native-drvs))
-
-         (define %build-target-inputs
-           ',(map (match-lambda
-                   ((name (? derivation? drv) sub ...)
-                    `(,name . ,(apply derivation->output-path drv sub)))
-                   ((name (? package? pkg) sub ...)
-                    (let ((drv (package-cross-derivation store pkg
-                                                         target system)))
-                      `(,name . ,(apply derivation->output-path drv sub))))
-                   ((name path)
-                    `(,name . ,path)))
-                  target-drvs))
-
-         (gnu-build #:source ,(match (assoc-ref native-drvs "source")
-                                (((? derivation? source))
-                                 (derivation->output-path source))
-                                ((source)
-                                 source)
-                                (source
-                                 source))
-                    #:system ,system
-                    #:build ,build
-                    #:target ,target
-                    #:outputs %outputs
-                    #:inputs %build-target-inputs
-                    #:native-inputs %build-host-inputs
-                    #:search-paths ',(map search-path-specification->sexp
+    #~(begin
+        (use-modules #$@modules)
+
+        (define %build-host-inputs
+          (map (lambda (tuple)
+                 (apply cons tuple))
+               '#+build-inputs))
+
+        (define %build-target-inputs
+          (map (lambda (tuple)
+                 (apply cons tuple))
+               (append '#$host-inputs '#+target-inputs)))
+
+        (define %outputs
+          (list #$@(map (lambda (name)
+                          #~(cons #$name
+                                  (ungexp output name)))
+                        outputs)))
+
+        (gnu-build #:source #+source
+                   #:system #$system
+                   #:build #$build
+                   #:target #$target
+                   #:outputs %outputs
+                   #:inputs %build-target-inputs
+                   #:native-inputs %build-host-inputs
+                   #:search-paths '#$(map search-path-specification->sexp
                                           search-paths)
-                    #:native-search-paths ',(map
+                   #:native-search-paths '#$(map
                                              search-path-specification->sexp
                                              native-search-paths)
-                    #:phases ,phases
-                    #:locale ,locale
-                    #:bootstrap-scripts ,bootstrap-scripts
-                    #:configure-flags ,configure-flags
-                    #:make-flags ,make-flags
-                    #:out-of-source? ,out-of-source?
-                    #:tests? ,tests?
-                    #:test-target ,test-target
-                    #:parallel-build? ,parallel-build?
-                    #:parallel-tests? ,parallel-tests?
-                    #:patch-shebangs? ,patch-shebangs?
-                    #:strip-binaries? ,strip-binaries?
-                    #:validate-runpath? ,validate-runpath?
-                    #:make-dynamic-linker-cache? ,make-dynamic-linker-cache?
-                    #:license-file-regexp ,license-file-regexp
-                    #:strip-flags ,strip-flags
-                    #:strip-directories ,strip-directories))))
-
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
-
-  (build-expression->derivation store name builder
-                                #:system system
-                                #:inputs (append native-drvs target-drvs)
-                                #:outputs outputs
-                                #:modules imported-modules
-                                #:substitutable? substitutable?
-
-                                #:allowed-references
-                                (and allowed-references
-                                     (map canonicalize-reference
-                                          allowed-references))
-                                #:disallowed-references
-                                (and disallowed-references
-                                     (map canonicalize-reference
-                                          disallowed-references))
-                                #:guile-for-build guile-for-build))
+                   #:phases #$phases
+                   #:locale #$locale
+                   #:bootstrap-scripts #$bootstrap-scripts
+                   #:configure-flags #$configure-flags
+                   #:make-flags #$make-flags
+                   #:out-of-source? #$out-of-source?
+                   #:tests? #$tests?
+                   #:test-target #$test-target
+                   #:parallel-build? #$parallel-build?
+                   #:parallel-tests? #$parallel-tests?
+                   #:patch-shebangs? #$patch-shebangs?
+                   #:license-file-regexp #$license-file-regexp
+                   #:strip-binaries? #$strip-binaries?
+                   #:validate-runpath? #$validate-runpath?
+                   #:make-dynamic-linker-cache? #$make-dynamic-linker-cache?
+                   #:license-file-regexp #$license-file-regexp
+                   #:strip-flags #$strip-flags
+                   #:strip-directories #$strip-directories)))
+
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name builder
+                      #:system system
+                      #:target target
+                      #:modules imported-modules
+                      #:substitutable? substitutable?
+                      #:allowed-references allowed-references
+                      #:disallowed-references disallowed-references
+                      #:guile-for-build guile)))
 
 (define gnu-build-system
   (build-system
diff --git a/guix/build-system/go.scm b/guix/build-system/go.scm
index 0e2c1cd2ee..4c8156e73c 100644
--- a/guix/build-system/go.scm
+++ b/guix/build-system/go.scm
@@ -2,6 +2,7 @@
 ;;; Copyright © 2016 Petter <petter@mykolab.ch>
 ;;; Copyright © 2017 Leo Famulari <leo@famulari.name>
 ;;; Copyright © 2020 Jakub Kądziołka <kuba@kadziolka.net>
+;;; Copyright © 2021 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -20,7 +21,9 @@
 
 (define-module (guix build-system go)
   #:use-module (guix utils)
-  #:use-module (guix derivations)
+  #:use-module (guix gexp)
+  #:use-module (guix store)
+  #:use-module (guix monads)
   #:use-module (guix search-paths)
   #:use-module (guix build-system)
   #:use-module (guix build-system gnu)
@@ -88,7 +91,7 @@ it, defaulting to full VERSION if a pseudo-version pattern is not recognized."
                 #:rest arguments)
   "Return a bag for NAME."
   (define private-keywords
-    '(#:source #:target #:go #:inputs #:native-inputs))
+    '(#:target #:go #:inputs #:native-inputs))
 
   (and (not target)                               ;XXX: no cross-compilation
        (bag
@@ -107,8 +110,9 @@ it, defaulting to full VERSION if a pseudo-version pattern is not recognized."
          (build go-build)
          (arguments (strip-keyword-arguments private-keywords arguments)))))
 
-(define* (go-build store name inputs
+(define* (go-build name inputs
                    #:key
+                   source
                    (phases '(@ (guix build go-build-system)
                                %standard-phases))
                    (outputs '("out"))
@@ -126,45 +130,29 @@ it, defaulting to full VERSION if a pseudo-version pattern is not recognized."
                               (guix build union)
                               (guix build utils))))
   (define builder
-   `(begin
-      (use-modules ,@modules)
-      (go-build #:name ,name
-                #:source ,(match (assoc-ref inputs "source")
-                                 (((? derivation? source))
-                                  (derivation->output-path source))
-                                 ((source)
-                                  source)
-                                 (source
-                                  source))
-                #:system ,system
-                #:phases ,phases
-                #:outputs %outputs
-                #:search-paths ',(map search-path-specification->sexp
-                                      search-paths)
-                #:install-source? ,install-source?
-                #:import-path ,import-path
-                #:unpack-path ,unpack-path
-                #:build-flags ,build-flags
-                #:tests? ,tests?
-                #:allow-go-reference? ,allow-go-reference?
-                #:inputs %build-inputs)))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
+          (go-build #:name #$name
+                    #:source #+source
+                    #:system #$system
+                    #:phases #$phases
+                    #:outputs #$(outputs->gexp outputs)
+                    #:search-paths '#$(map search-path-specification->sexp
+                                           search-paths)
+                    #:install-source? #$install-source?
+                    #:import-path #$import-path
+                    #:unpack-path #$unpack-path
+                    #:build-flags #$build-flags
+                    #:tests? #$tests?
+                    #:allow-go-reference? #$allow-go-reference?
+                    #:inputs #$(input-tuples->gexp inputs)))))
 
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system
-                             #:graft? #f)))))
-
-  (build-expression->derivation store name builder
-                                #:inputs inputs
-                                #:system system
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build))
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name builder
+                      #:system system
+                      #:guile-for-build guile)))
 
 (define go-build-system
   (build-system
diff --git a/guix/build-system/guile.scm b/guix/build-system/guile.scm
index 45e735b987..f64f214675 100644
--- a/guix/build-system/guile.scm
+++ b/guix/build-system/guile.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2018, 2019 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2018, 2019, 2021 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -20,7 +20,8 @@
   #:use-module (guix store)
   #:use-module (guix utils)
   #:use-module (guix packages)
-  #:use-module (guix derivations)
+  #:use-module (guix monads)
+  #:use-module (guix gexp)
   #:use-module (guix search-paths)
   #:use-module (guix build-system)
   #:use-module (guix build-system gnu)
@@ -75,7 +76,7 @@
   ;; denominator between Guile 2.0 and 2.2.
   ''("-Wunbound-variable" "-Warity-mismatch" "-Wformat"))
 
-(define* (guile-build store name inputs
+(define* (guile-build name inputs
                       #:key source
                       (guile #f)
                       (phases '%standard-phases)
@@ -91,47 +92,34 @@
                                  (guix build utils))))
   "Build SOURCE using Guile taken from the native inputs, and with INPUTS."
   (define builder
-    `(begin
-       (use-modules ,@modules)
-       (guile-build #:name ,name
-                    #:source ,(match (assoc-ref inputs "source")
-                                (((? derivation? source))
-                                 (derivation->output-path source))
-                                ((source)
-                                 source)
-                                (source
-                                 source))
-                    #:source-directory ,source-directory
-                    #:scheme-file-regexp ,scheme-file-regexp
-                    #:not-compiled-file-regexp ,not-compiled-file-regexp
-                    #:compile-flags ,compile-flags
-                    #:phases ,phases
-                    #:system ,system
-                    #:outputs %outputs
-                    #:search-paths ',(map search-path-specification->sexp
-                                          search-paths)
-                    #:inputs %build-inputs)))
-
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
-
-  (build-expression->derivation store name builder
-                                #:inputs inputs
-                                #:system system
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build))
-
-(define* (guile-cross-build store name
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
+
+          (guile-build #:name #$name
+                       #:source #+source
+                       #:source-directory #$source-directory
+                       #:scheme-file-regexp #$scheme-file-regexp
+                       #:not-compiled-file-regexp #$not-compiled-file-regexp
+                       #:compile-flags #$compile-flags
+                       #:phases #$phases
+                       #:system #$system
+                       #:outputs #$(outputs->gexp outputs)
+                       #:inputs #$(input-tuples->gexp inputs)
+                       #:search-paths '#$(map search-path-specification->sexp
+                                              search-paths)))))
+
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name builder
+                      #:system system
+                      #:target #f
+                      #:guile-for-build guile)))
+
+(define* (guile-cross-build name
                             #:key
                             (system (%current-system)) target
-                            native-drvs target-drvs
+                            build-inputs target-inputs host-inputs
                             (guile #f)
                             source
                             (outputs '("out"))
@@ -146,68 +134,42 @@
                             (modules '((guix build guile-build-system)
                                        (guix build utils))))
   (define builder
-    `(begin
-       (use-modules ,@modules)
-
-       (let ()
-         (define %build-host-inputs
-           ',(map (match-lambda
-                    ((name (? derivation? drv) sub ...)
-                     `(,name . ,(apply derivation->output-path drv sub)))
-                    ((name path)
-                     `(,name . ,path)))
-                  native-drvs))
-
-         (define %build-target-inputs
-           ',(map (match-lambda
-                    ((name (? derivation? drv) sub ...)
-                     `(,name . ,(apply derivation->output-path drv sub)))
-                    ((name (? package? pkg) sub ...)
-                     (let ((drv (package-cross-derivation store pkg
-                                                          target system)))
-                       `(,name . ,(apply derivation->output-path drv sub))))
-                    ((name path)
-                     `(,name . ,path)))
-                  target-drvs))
-
-         (guile-build #:source ,(match (assoc-ref native-drvs "source")
-                                  (((? derivation? source))
-                                   (derivation->output-path source))
-                                  ((source)
-                                   source)
-                                  (source
-                                   source))
-                      #:system ,system
-                      #:target ,target
-                      #:outputs %outputs
-                      #:source-directory ,source-directory
-                      #:not-compiled-file-regexp ,not-compiled-file-regexp
-                      #:compile-flags ,compile-flags
-                      #:inputs %build-target-inputs
-                      #:native-inputs %build-host-inputs
-                      #:search-paths ',(map search-path-specification->sexp
-                                            search-paths)
-                      #:native-search-paths ',(map
-                                               search-path-specification->sexp
-                                               native-search-paths)
-                      #:phases ,phases))))
-
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
-
-  (build-expression->derivation store name builder
-                                #:system system
-                                #:inputs (append native-drvs target-drvs)
-                                #:outputs outputs
-                                #:modules imported-modules
-                                #:substitutable? substitutable?
-                                #:guile-for-build guile-for-build))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
+
+          (define %build-host-inputs
+            #+(input-tuples->gexp build-inputs))
+
+          (define %build-target-inputs
+            (append #$(input-tuples->gexp host-inputs)
+                    #+(input-tuples->gexp target-inputs)))
+
+          (define %outputs
+            #$(outputs->gexp outputs))
+
+          (guile-build #:source #+source
+                       #:system #$system
+                       #:target #$target
+                       #:outputs %outputs
+                       #:source-directory #$source-directory
+                       #:not-compiled-file-regexp #$not-compiled-file-regexp
+                       #:compile-flags #$compile-flags
+                       #:inputs %build-target-inputs
+                       #:native-inputs %build-host-inputs
+                       #:search-paths '#$(map search-path-specification->sexp
+                                              search-paths)
+                       #:native-search-paths '#$(map
+                                                 search-path-specification->sexp
+                                                 native-search-paths)
+                       #:phases #$phases))))
+
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name builder
+                      #:system system
+                      #:target target
+                      #:guile-for-build guile)))
 
 (define guile-build-system
   (build-system
diff --git a/guix/build-system/haskell.scm b/guix/build-system/haskell.scm
index 18a584f782..b7ee72557c 100644
--- a/guix/build-system/haskell.scm
+++ b/guix/build-system/haskell.scm
@@ -2,6 +2,7 @@
 ;;; Copyright © 2015 Federico Beffa <beffa@fbengineering.ch>
 ;;; Copyright © 2020 Timothy Sample <samplet@ngyro.com>
 ;;; Copyright © 2020 Simon Tournier <zimon.toutoune@gmail.com>
+;;; Copyright © 2021 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -22,7 +23,8 @@
   #:use-module (guix store)
   #:use-module (guix utils)
   #:use-module (guix packages)
-  #:use-module (guix derivations)
+  #:use-module (guix gexp)
+  #:use-module (guix monads)
   #:use-module (guix download)
   #:use-module (guix search-paths)
   #:use-module (guix build-system)
@@ -116,7 +118,7 @@ version REVISION."
                                (cons name propagated-names))))))
                        extra-directories))))))))
 
-(define* (haskell-build store name inputs
+(define* (haskell-build name inputs
                         #:key source
                         (haddock? #t)
                         (haddock-flags ''())
@@ -139,50 +141,33 @@ version REVISION."
   "Build SOURCE using HASKELL, and with INPUTS.  This assumes that SOURCE
 provides a 'Setup.hs' file as its build system."
   (define builder
-    `(begin
-       (use-modules ,@modules)
-       (haskell-build #:name ,name
-                      #:source ,(match (assoc-ref inputs "source")
-                                  (((? derivation? source))
-                                   (derivation->output-path source))
-                                  ((source)
-                                   source)
-                                  (source
-                                   source))
-                      #:cabal-revision ,(match (assoc-ref inputs
-                                                          "cabal-revision")
-                                          (((? derivation? revision))
-                                           (derivation->output-path revision))
-                                          (revision revision))
-                      #:configure-flags ,configure-flags
-                      #:extra-directories ,extra-directories
-                      #:haddock-flags ,haddock-flags
-                      #:system ,system
-                      #:test-target ,test-target
-                      #:tests? ,tests?
-                      #:parallel-build? ,parallel-build?
-                      #:haddock? ,haddock?
-                      #:phases ,phases
-                      #:outputs %outputs
-                      #:search-paths ',(map search-path-specification->sexp
-                                            search-paths)
-                      #:inputs %build-inputs)))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
 
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
+          (haskell-build #:name #$name
+                         #:source #+source
+                         #:cabal-revision #$(assoc-ref inputs
+                                                       "cabal-revision")
+                         #:configure-flags #$configure-flags
+                         #:extra-directories #$extra-directories
+                         #:haddock-flags #$haddock-flags
+                         #:system #$system
+                         #:test-target #$test-target
+                         #:tests? #$tests?
+                         #:parallel-build? #$parallel-build?
+                         #:haddock? #$haddock?
+                         #:phases #$phases
+                         #:outputs #$(outputs->gexp outputs)
+                         #:search-paths '#$(map search-path-specification->sexp
+                                                search-paths)
+                         #:inputs #$(input-tuples->gexp inputs)))))
 
-  (build-expression->derivation store name builder
-                                #:inputs inputs
-                                #:system system
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build))
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name builder
+                      #:system system
+                      #:guile-for-build guile)))
 
 (define haskell-build-system
   (build-system
diff --git a/guix/build-system/julia.scm b/guix/build-system/julia.scm
index 63cb7cd864..6e006af527 100644
--- a/guix/build-system/julia.scm
+++ b/guix/build-system/julia.scm
@@ -1,5 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2019 Nicolò Balzarotti <nicolo@nixo.xyz>
+;;; Copyright © 2021 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -20,7 +21,8 @@
   #:use-module (guix store)
   #:use-module (guix utils)
   #:use-module (guix packages)
-  #:use-module (guix derivations)
+  #:use-module (guix gexp)
+  #:use-module (guix monads)
   #:use-module (guix search-paths)
   #:use-module (guix build-system)
   #:use-module (guix build-system gnu)
@@ -73,7 +75,7 @@
          (build julia-build)
          (arguments (strip-keyword-arguments private-keywords arguments)))))
 
-(define* (julia-build store name inputs
+(define* (julia-build name inputs
                       #:key source
                       (tests? #t)
                       (phases '(@ (guix build julia-build-system)
@@ -88,40 +90,25 @@
                                  (guix build utils))))
   "Build SOURCE using Julia, and with INPUTS."
   (define builder
-    `(begin
-       (use-modules ,@modules)
-       (julia-build #:name ,name
-                    #:source ,(match (assoc-ref inputs "source")
-                                (((? derivation? source))
-                                 (derivation->output-path source))
-                                ((source)
-                                 source)
-                                (source
-                                 source))
-                    #:system ,system
-                    #:tests? ,tests?
-                    #:phases ,phases
-                    #:outputs %outputs
-                    #:search-paths ',(map search-path-specification->sexp
-                                          search-paths)
-                    #:inputs %build-inputs
-                    #:julia-package-name ,julia-package-name)))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
+          (julia-build #:name #$name
+                       #:source #+source
+                       #:system #$system
+                       #:tests? #$tests?
+                       #:phases #$phases
+                       #:outputs #$(outputs->gexp outputs)
+                       #:search-paths '#$(map search-path-specification->sexp
+                                              search-paths)
+                       #:inputs #$(input-tuples->gexp inputs)
+                       #:julia-package-name #$julia-package-name))))
 
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
-
-  (build-expression->derivation store name builder
-                                #:inputs inputs
-                                #:system system
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build))
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name builder
+                      #:system system
+                      #:guile-for-build guile)))
 
 (define julia-build-system
   (build-system
diff --git a/guix/build-system/linux-module.scm b/guix/build-system/linux-module.scm
index fc3d959ce7..3a29a93bd7 100644
--- a/guix/build-system/linux-module.scm
+++ b/guix/build-system/linux-module.scm
@@ -1,6 +1,7 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2019 Danny Milosavljevic <dannym@scratchpost.org>
 ;;; Copyright © 2020 Mathieu Othacehe <m.othacehe@gmail.com>
+;;; Copyright © 2021 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -20,7 +21,8 @@
 (define-module (guix build-system linux-module)
   #:use-module (guix store)
   #:use-module (guix utils)
-  #:use-module (guix derivations)
+  #:use-module (guix gexp)
+  #:use-module (guix monads)
   #:use-module (guix search-paths)
   #:use-module (guix build-system)
   #:use-module (guix build-system gnu)
@@ -114,7 +116,7 @@
                 #:rest arguments)
   "Return a bag for NAME."
   (define private-keywords
-    `(#:source #:target #:gcc #:kmod #:linux #:inputs #:native-inputs
+    `(#:target #:gcc #:kmod #:linux #:inputs #:native-inputs
       ,@(if target '() '(#:target))))
 
   (bag
@@ -148,9 +150,9 @@
     (build (if target linux-module-build-cross linux-module-build))
     (arguments (strip-keyword-arguments private-keywords arguments))))
 
-(define* (linux-module-build store name inputs
+(define* (linux-module-build name inputs
                              #:key
-                             target
+                             source target
                              (search-paths '())
                              (tests? #t)
                              (phases '(@ (guix build linux-module-build-system)
@@ -166,48 +168,34 @@
                                         (guix build utils))))
   "Build SOURCE using LINUX, and with INPUTS."
   (define builder
-    `(begin
-       (use-modules ,@modules)
-       (linux-module-build #:name ,name
-                     #:source ,(match (assoc-ref inputs "source")
-                                      (((? derivation? source))
-                                       (derivation->output-path source))
-                                      ((source)
-                                       source)
-                                      (source
-                                        source))
-                     #:search-paths ',(map search-path-specification->sexp
-                                           search-paths)
-                     #:phases ,phases
-                     #:system ,system
-                     #:target ,target
-                     #:arch ,(system->arch (or target system))
-                     #:tests? ,tests?
-                     #:outputs %outputs
-                     #:make-flags ,make-flags
-                     #:inputs %build-inputs)))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
+          (linux-module-build #:name #$name
+                              #:source #+source
+                              #:search-paths '#$(map search-path-specification->sexp
+                                                     search-paths)
+                              #:phases #$phases
+                              #:system #$system
+                              #:target #$target
+                              #:arch #$(system->arch (or target system))
+                              #:tests? #$tests?
+                              #:outputs #$(outputs->gexp outputs)
+                              #:make-flags #$make-flags
+                              #:inputs #$(input-tuples->gexp inputs)))))
 
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
-
-  (build-expression->derivation store name builder
-                                #:system system
-                                #:inputs inputs
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build
-                                #:substitutable? substitutable?))
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name builder
+                      #:system system
+                      #:guile-for-build guile
+                      #:substitutable? substitutable?)))
 
 (define* (linux-module-build-cross
-          store name
+          name
           #:key
-          target native-drvs target-drvs
+          source target
+          build-inputs target-inputs host-inputs
           (guile #f)
           (outputs '("out"))
           (make-flags ''())
@@ -223,70 +211,42 @@
           (modules '((guix build linux-module-build-system)
                      (guix build utils))))
   (define builder
-    `(begin
-       (use-modules ,@modules)
-       (let ()
-         (define %build-host-inputs
-           ',(map (match-lambda
-                    ((name (? derivation? drv) sub ...)
-                     `(,name . ,(apply derivation->output-path drv sub)))
-                    ((name path)
-                     `(,name . ,path)))
-                  native-drvs))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
 
-         (define %build-target-inputs
-           ',(map (match-lambda
-                    ((name (? derivation? drv) sub ...)
-                     `(,name . ,(apply derivation->output-path drv sub)))
-                    ((name (? package? pkg) sub ...)
-                     (let ((drv (package-cross-derivation store pkg
-                                                          target system)))
-                       `(,name . ,(apply derivation->output-path drv sub))))
-                    ((name path)
-                     `(,name . ,path)))
-                  target-drvs))
+          (define %build-host-inputs
+            '#+(input-tuples->gexp build-inputs))
 
-         (linux-module-build #:name ,name
-                             #:source ,(match (assoc-ref native-drvs "source")
-                                         (((? derivation? source))
-                                          (derivation->output-path source))
-                                         ((source)
-                                          source)
-                                         (source
-                                          source))
-                             #:system ,system
-                             #:target ,target
-                             #:arch ,(system->arch (or target system))
-                             #:outputs %outputs
-                             #:make-flags ,make-flags
-                             #:inputs %build-target-inputs
-                             #:native-inputs %build-host-inputs
-                             #:search-paths
-                             ',(map search-path-specification->sexp
-                                    search-paths)
-                             #:native-search-paths
-                             ',(map
-                                search-path-specification->sexp
-                                native-search-paths)
-                             #:phases ,phases
-                             #:tests? ,tests?))))
+          (define %build-target-inputs
+            (append #$(input-tuples->gexp host-inputs)
+                    #+(input-tuples->gexp target-inputs)))
 
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
+          (linux-module-build #:name #$name
+                              #:source #+source
+                              #:system #$system
+                              #:target #$target
+                              #:arch #$(system->arch (or target system))
+                              #:outputs #$(outputs->gexp outputs)
+                              #:make-flags #$make-flags
+                              #:inputs %build-target-inputs
+                              #:native-inputs %build-host-inputs
+                              #:search-paths
+                              '#$(map search-path-specification->sexp
+                                      search-paths)
+                              #:native-search-paths
+                              '#$(map
+                                  search-path-specification->sexp
+                                  native-search-paths)
+                              #:phases #$phases
+                              #:tests? #$tests?))))
 
-  (build-expression->derivation store name builder
-                                #:system system
-                                #:inputs (append native-drvs target-drvs)
-                                #:outputs outputs
-                                #:modules imported-modules
-                                #:guile-for-build guile-for-build
-                                #:substitutable? substitutable?))
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name builder
+                      #:system system
+                      #:guile-for-build guile
+                      #:substitutable? substitutable?)))
 
 (define linux-module-build-system
   (build-system
diff --git a/guix/build-system/maven.scm b/guix/build-system/maven.scm
index 2dceefccc1..04358e6240 100644
--- a/guix/build-system/maven.scm
+++ b/guix/build-system/maven.scm
@@ -1,5 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2020 Julien Lepiller <julien@lepiller.eu>
+;;; Copyright © 2021 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -19,7 +20,8 @@
 (define-module (guix build-system maven)
   #:use-module (guix store)
   #:use-module (guix utils)
-  #:use-module (guix derivations)
+  #:use-module (guix gexp)
+  #:use-module (guix monads)
   #:use-module (guix search-paths)
   #:use-module (guix build-system)
   #:use-module (guix build-system gnu)
@@ -119,7 +121,7 @@
                 #:rest arguments)
   "Return a bag for NAME."
   (define private-keywords
-    '(#:source #:target #:jdk #:maven #:maven-plugins #:inputs #:native-inputs))
+    '(#:target #:jdk #:maven #:maven-plugins #:inputs #:native-inputs))
 
   (and (not target)                               ;XXX: no cross-compilation
        (bag
@@ -140,70 +142,56 @@
          (build maven-build)
          (arguments (strip-keyword-arguments private-keywords arguments)))))
 
-(define* (maven-build store name inputs
-                       #:key (guile #f)
-                       (outputs '("out"))
-                       (search-paths '())
-                       (out-of-source? #t)
-                       (validate-runpath? #t)
-                       (patch-shebangs? #t)
-                       (strip-binaries? #t)
-                       (exclude %default-exclude)
-                       (local-packages '())
-                       (tests? #t)
-                       (strip-flags ''("--strip-debug"))
-                       (strip-directories ''("lib" "lib64" "libexec"
-                                             "bin" "sbin"))
-                       (phases '(@ (guix build maven-build-system)
-                                   %standard-phases))
-                       (system (%current-system))
-                       (imported-modules %maven-build-system-modules)
-                       (modules '((guix build maven-build-system)
-                                  (guix build maven pom)
-                                  (guix build utils))))
+(define* (maven-build name inputs
+                      #:key
+                      source (guile #f)
+                      (outputs '("out"))
+                      (search-paths '())
+                      (out-of-source? #t)
+                      (validate-runpath? #t)
+                      (patch-shebangs? #t)
+                      (strip-binaries? #t)
+                      (exclude %default-exclude)
+                      (local-packages '())
+                      (tests? #t)
+                      (strip-flags ''("--strip-debug"))
+                      (strip-directories ''("lib" "lib64" "libexec"
+                                            "bin" "sbin"))
+                      (phases '(@ (guix build maven-build-system)
+                                  %standard-phases))
+                      (system (%current-system))
+                      (imported-modules %maven-build-system-modules)
+                      (modules '((guix build maven-build-system)
+                                 (guix build maven pom)
+                                 (guix build utils))))
   "Build SOURCE using PATCHELF, and with INPUTS. This assumes that SOURCE
 provides its own binaries."
   (define builder
-    `(begin
-       (use-modules ,@modules)
-       (maven-build #:source ,(match (assoc-ref inputs "source")
-                                 (((? derivation? source))
-                                  (derivation->output-path source))
-                                 ((source)
-                                  source)
-                                 (source
-                                  source))
-                     #:system ,system
-                     #:outputs %outputs
-                     #:inputs %build-inputs
-                     #:search-paths ',(map search-path-specification->sexp
-                                           search-paths)
-                     #:phases ,phases
-                     #:exclude (quote ,exclude)
-                     #:local-packages (quote ,local-packages)
-                     #:tests? ,tests?
-                     #:out-of-source? ,out-of-source?
-                     #:validate-runpath? ,validate-runpath?
-                     #:patch-shebangs? ,patch-shebangs?
-                     #:strip-binaries? ,strip-binaries?
-                     #:strip-flags ,strip-flags
-                     #:strip-directories ,strip-directories)))
-
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
-
-  (build-expression->derivation store name builder
-                                #:system system
-                                #:inputs inputs
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
+          (maven-build #:source #+source
+                       #:system #$system
+                       #:outputs #$(outputs->gexp outputs)
+                       #:inputs #$(input-tuples->gexp inputs)
+                       #:search-paths '#$(map search-path-specification->sexp
+                                              search-paths)
+                       #:phases #$phases
+                       #:exclude '#$exclude
+                       #:local-packages '#$local-packages
+                       #:tests? #$tests?
+                       #:out-of-source? #$out-of-source?
+                       #:validate-runpath? #$validate-runpath?
+                       #:patch-shebangs? #$patch-shebangs?
+                       #:strip-binaries? #$strip-binaries?
+                       #:strip-flags #$strip-flags
+                       #:strip-directories #$strip-directories))))
+
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name builder
+                      #:system system
+                      #:guile-for-build guile)))
 
 (define maven-build-system
   (build-system
diff --git a/guix/build-system/meson.scm b/guix/build-system/meson.scm
index e04223381e..19fbe54758 100644
--- a/guix/build-system/meson.scm
+++ b/guix/build-system/meson.scm
@@ -1,6 +1,7 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2017 Peter Mikkelsen <petermikkelsen10@gmail.com>
 ;;; Copyright © 2018, 2019 Marius Bakke <mbakke@fastmail.com>
+;;; Copyright © 2021 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -18,9 +19,10 @@
 ;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
 
 (define-module (guix build-system meson)
-  #:use-module (guix store)
+  #:use-module (guix gexp)
   #:use-module (guix utils)
-  #:use-module (guix derivations)
+  #:use-module (guix store)
+  #:use-module (guix monads)
   #:use-module (guix search-paths)
   #:use-module (guix build-system)
   #:use-module (guix build-system gnu)
@@ -66,7 +68,7 @@
                 #:rest arguments)
   "Return a bag for NAME."
   (define private-keywords
-    `(#:source #:meson #:ninja #:inputs #:native-inputs #:outputs #:target))
+    `(#:meson #:ninja #:inputs #:native-inputs #:outputs #:target))
 
   (and (not target) ;; TODO: add support for cross-compilation.
        (bag
@@ -85,8 +87,9 @@
          (build meson-build)
          (arguments (strip-keyword-arguments private-keywords arguments)))))
 
-(define* (meson-build store name inputs
-                      #:key (guile #f)
+(define* (meson-build name inputs
+                      #:key
+                      guile source
                       (outputs '("out"))
                       (configure-flags ''())
                       (search-paths '())
@@ -114,76 +117,48 @@
                       disallowed-references)
   "Build SOURCE using MESON, and with INPUTS, assuming that SOURCE
 has a 'meson.build' file."
-
-  ;; TODO: Copied from build-system/gnu, factorize this!
-  (define canonicalize-reference
-    (match-lambda
-     ((? package? p)
-      (derivation->output-path (package-derivation store p system
-                                                   #:graft? #f)))
-     (((? package? p) output)
-      (derivation->output-path (package-derivation store p system
-                                                   #:graft? #f)
-                               output))
-     ((? string? output)
-      output)))
-
   (define builder
-    `(let ((build-phases (if ,glib-or-gtk?
-                             ,phases
-                             (modify-phases ,phases
-                               (delete 'glib-or-gtk-compile-schemas)
-                               (delete 'glib-or-gtk-wrap)))))
-       (use-modules ,@modules)
-       (meson-build #:source ,(match (assoc-ref inputs "source")
-                                (((? derivation? source))
-                                 (derivation->output-path source))
-                                ((source)
-                                 source)
-                                (source
-                                 source))
-                    #:system ,system
-                    #:outputs %outputs
-                    #:inputs %build-inputs
-                    #:search-paths ',(map search-path-specification->sexp
-                                          search-paths)
-                    #:phases build-phases
-                    #:configure-flags ,configure-flags
-                    #:build-type ,build-type
-                    #:tests? ,tests?
-                    #:test-target ,test-target
-                    #:parallel-build? ,parallel-build?
-                    #:parallel-tests? ,parallel-tests?
-                    #:validate-runpath? ,validate-runpath?
-                    #:patch-shebangs? ,patch-shebangs?
-                    #:strip-binaries? ,strip-binaries?
-                    #:strip-flags ,strip-flags
-                    #:strip-directories ,strip-directories
-                    #:elf-directories ,elf-directories)))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
+
+          (define build-phases
+            #$(if glib-or-gtk?
+                  phases
+                  #~(modify-phases #$phases
+                      (delete 'glib-or-gtk-compile-schemas)
+                      (delete 'glib-or-gtk-wrap))))
 
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
+          #$(with-build-variables inputs outputs
+              #~(meson-build #:source #+source
+                             #:system #$system
+                             #:outputs %outputs
+                             #:inputs %build-inputs
+                             #:search-paths '#$(map search-path-specification->sexp
+                                                    search-paths)
+                             #:phases build-phases
+                             #:configure-flags #$configure-flags
+                             #:build-type #$build-type
+                             #:tests? #$tests?
+                             #:test-target #$test-target
+                             #:parallel-build? #$parallel-build?
+                             #:parallel-tests? #$parallel-tests?
+                             #:validate-runpath? #$validate-runpath?
+                             #:patch-shebangs? #$patch-shebangs?
+                             #:strip-binaries? #$strip-binaries?
+                             #:strip-flags #$strip-flags
+                             #:strip-directories #$strip-directories
+                             #:elf-directories #$elf-directories)))))
 
-  (build-expression->derivation store name builder
-                                #:system system
-                                #:inputs inputs
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build
-                                #:allowed-references
-                                (and allowed-references
-                                     (map canonicalize-reference
-                                          allowed-references))
-                                #:disallowed-references
-                                (and disallowed-references
-                                     (map canonicalize-reference
-                                          disallowed-references))))
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name builder
+                      #:system system
+                      #:target #f
+                      #:substitutable? substitutable?
+                      #:allowed-references allowed-references
+                      #:disallowed-references disallowed-references
+                      #:guile-for-build guile)))
 
 (define meson-build-system
   (build-system
diff --git a/guix/build-system/minify.scm b/guix/build-system/minify.scm
index 28a6781c06..751312a523 100644
--- a/guix/build-system/minify.scm
+++ b/guix/build-system/minify.scm
@@ -1,5 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2017, 2018 Ricardo Wurmus <rekado@elephly.net>
+;;; Copyright © 2021 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -20,7 +21,8 @@
   #:use-module (guix store)
   #:use-module (guix utils)
   #:use-module (guix packages)
-  #:use-module (guix derivations)
+  #:use-module (guix gexp)
+  #:use-module (guix monads)
   #:use-module (guix search-paths)
   #:use-module (guix build-system)
   #:use-module (guix build-system gnu)
@@ -54,7 +56,7 @@
                 #:rest arguments)
   "Return a bag for NAME."
   (define private-keywords
-    '(#:source #:target #:inputs #:native-inputs))
+    '(#:target #:inputs #:native-inputs))
 
   (bag
     (name name)
@@ -70,8 +72,9 @@
     (build minify-build)
     (arguments (strip-keyword-arguments private-keywords arguments))))
 
-(define* (minify-build store name inputs
+(define* (minify-build name inputs
                        #:key
+                       source
                        (javascript-files #f)
                        (phases '(@ (guix build minify-build-system)
                                    %standard-phases))
@@ -84,38 +87,23 @@
                                   (guix build utils))))
   "Build SOURCE with INPUTS."
   (define builder
-    `(begin
-       (use-modules ,@modules)
-       (minify-build #:name ,name
-                     #:source ,(match (assoc-ref inputs "source")
-                                 (((? derivation? source))
-                                  (derivation->output-path source))
-                                 ((source)
-                                  source)
-                                 (source
-                                  source))
-                     #:javascript-files ,javascript-files
-                     #:phases ,phases
-                     #:outputs %outputs
-                     #:search-paths ',(map search-path-specification->sexp
-                                           search-paths)
-                     #:inputs %build-inputs)))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
+          (minify-build #:name #$name
+                        #:source #+source
+                        #:javascript-files #$javascript-files
+                        #:phases #$phases
+                        #:outputs #$(outputs->gexp outputs)
+                        #:search-paths '#$(map search-path-specification->sexp
+                                               search-paths)
+                        #:inputs #$(input-tuples->gexp inputs)))))
 
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                               ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
-
-  (build-expression->derivation store name builder
-                                #:inputs inputs
-                                #:system system
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build))
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name builder
+                      #:system system
+                      #:guile-for-build guile)))
 
 (define minify-build-system
   (build-system
diff --git a/guix/build-system/node.scm b/guix/build-system/node.scm
index a8c5eed09b..c174da98aa 100644
--- a/guix/build-system/node.scm
+++ b/guix/build-system/node.scm
@@ -1,5 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2016 Jelle Licht <jlicht@fsfe.org>
+;;; Copyright © 2021 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -20,7 +21,8 @@
   #:use-module (guix store)
   #:use-module (guix utils)
   #:use-module (guix packages)
-  #:use-module (guix derivations)
+  #:use-module (guix gexp)
+  #:use-module (guix monads)
   #:use-module (guix search-paths)
   #:use-module (guix build-system)
   #:use-module (guix build-system gnu)
@@ -55,7 +57,7 @@ registry."
                 #:rest arguments)
   "Return a bag for NAME."
   (define private-keywords
-    '(#:source #:target #:node #:inputs #:native-inputs))
+    '(#:target #:node #:inputs #:native-inputs))
 
   (and (not target)                    ;XXX: no cross-compilation
        (bag
@@ -74,8 +76,9 @@ registry."
          (build node-build)
          (arguments (strip-keyword-arguments private-keywords arguments)))))
 
-(define* (node-build store name inputs
+(define* (node-build name inputs
                      #:key
+                     source
                      (npm-flags ''())
                      (tests? #t)
                      (phases '(@ (guix build node-build-system)
@@ -91,40 +94,25 @@ registry."
                                 (guix build utils))))
   "Build SOURCE using NODE and INPUTS."
   (define builder
-    `(begin
-       (use-modules ,@modules)
-       (node-build #:name ,name
-                   #:source ,(match (assoc-ref inputs "source")
-                               (((? derivation? source))
-                                (derivation->output-path source))
-                               ((source)
-                                source)
-                               (source
-                                source))
-                   #:system ,system
-                   #:npm-flags ,npm-flags
-                   #:tests? ,tests?
-                   #:phases ,phases
-                   #:outputs %outputs
-                   #:search-paths ',(map search-path-specification->sexp
-                                         search-paths)
-                   #:inputs %build-inputs)))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
+          (node-build #:name #$name
+                      #:source #+source
+                      #:system #$system
+                      #:npm-flags #$npm-flags
+                      #:tests? #$tests?
+                      #:phases #$phases
+                      #:outputs #$(outputs->gexp outputs)
+                      #:search-paths '#$(map search-path-specification->sexp
+                                             search-paths)
+                      #:inputs #$(input-tuples->gexp inputs)))))
 
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
-
-  (build-expression->derivation store name builder
-                                #:inputs inputs
-                                #:system system
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build))
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name builder
+                      #:system system
+                      #:guile-for-build guile)))
 
 (define node-build-system
   (build-system
diff --git a/guix/build-system/ocaml.scm b/guix/build-system/ocaml.scm
index 5513216c25..2f60f0d534 100644
--- a/guix/build-system/ocaml.scm
+++ b/guix/build-system/ocaml.scm
@@ -1,6 +1,7 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2016, 2017, 2018 Julien Lepiller <julien@lepiller.eu>
 ;;; Copyright © 2017 Ben Woodcroft <donttrustben@gmail.com>
+;;; Copyright © 2021 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -19,7 +20,7 @@
 (define-module (guix build-system ocaml)
   #:use-module (guix store)
   #:use-module (guix utils)
-  #:use-module (guix derivations)
+  #:use-module (guix gexp)
   #:use-module (guix search-paths)
   #:use-module (guix build-system)
   #:use-module (guix build-system gnu)
@@ -206,7 +207,7 @@ pre-defined variants."
                 #:rest arguments)
   "Return a bag for NAME."
   (define private-keywords
-    '(#:source #:target #:ocaml #:findlib #:inputs #:native-inputs))
+    '(#:target #:ocaml #:findlib #:inputs #:native-inputs))
 
   (and (not target)                               ;XXX: no cross-compilation
        (bag
@@ -226,8 +227,9 @@ pre-defined variants."
          (build ocaml-build)
          (arguments (strip-keyword-arguments private-keywords arguments)))))
 
-(define* (ocaml-build store name inputs
-                      #:key (guile #f)
+(define* (ocaml-build name inputs
+                      #:key
+                      guile source
                       (outputs '("out")) (configure-flags ''())
                       (search-paths '())
                       (make-flags ''())
@@ -253,51 +255,40 @@ pre-defined variants."
   "Build SOURCE using OCAML, and with INPUTS. This assumes that SOURCE
 provides a 'setup.ml' file as its build system."
   (define builder
-    `(begin
-       (use-modules ,@modules)
-       (ocaml-build #:source ,(match (assoc-ref inputs "source")
-                                (((? derivation? source))
-                                 (derivation->output-path source))
-                                ((source)
-                                 source)
-                                (source
-                                 source))
-                    #:system ,system
-                    #:outputs %outputs
-                    #:inputs %build-inputs
-                    #:search-paths ',(map search-path-specification->sexp
-                                          search-paths)
-                    #:phases ,phases
-                    #:configure-flags ,configure-flags
-                    #:test-flags ,test-flags
-                    #:make-flags ,make-flags
-                    #:build-flags ,build-flags
-                    #:out-of-source? ,out-of-source?
-                    #:use-make? ,use-make?
-                    #:tests? ,tests?
-                    #:test-target ,test-target
-                    #:install-target ,install-target
-                    #:validate-runpath? ,validate-runpath?
-                    #:patch-shebangs? ,patch-shebangs?
-                    #:strip-binaries? ,strip-binaries?
-                    #:strip-flags ,strip-flags
-                    #:strip-directories ,strip-directories)))
-
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
-
-  (build-expression->derivation store name builder
-                                #:system system
-                                #:inputs inputs
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
+          (ocaml-build #:source #$source
+                       #:system #$system
+                       #:outputs (list #$@(map (lambda (name)
+                                                 #~(cons #$name
+                                                         (ungexp output name)))
+                                               outputs))
+                       #:inputs (map (lambda (tuple)
+                                       (apply cons tuple))
+                                     '#$inputs)
+                       #:search-paths '#$(map search-path-specification->sexp
+                                              search-paths)
+                       #:phases #$phases
+                       #:configure-flags #$configure-flags
+                       #:test-flags #$test-flags
+                       #:make-flags #$make-flags
+                       #:build-flags #$build-flags
+                       #:out-of-source? #$out-of-source?
+                       #:use-make? #$use-make?
+                       #:tests? #$tests?
+                       #:test-target #$test-target
+                       #:install-target #$install-target
+                       #:validate-runpath? #$validate-runpath?
+                       #:patch-shebangs? #$patch-shebangs?
+                       #:strip-binaries? #$strip-binaries?
+                       #:strip-flags #$strip-flags
+                       #:strip-directories #$strip-directories))))
+
+  (gexp->derivation name builder
+                    #:system system
+                    #:target #f
+                    #:guile-for-build guile))
 
 (define ocaml-build-system
   (build-system
diff --git a/guix/build-system/perl.scm b/guix/build-system/perl.scm
index 06af1dd20e..32045ef6de 100644
--- a/guix/build-system/perl.scm
+++ b/guix/build-system/perl.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2013, 2014, 2015 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2013, 2014, 2015, 2021 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -19,6 +19,8 @@
 (define-module (guix build-system perl)
   #:use-module (guix store)
   #:use-module (guix utils)
+  #:use-module (guix gexp)
+  #:use-module (guix monads)
   #:use-module (guix derivations)
   #:use-module (guix search-paths)
   #:use-module (guix build-system)
@@ -57,7 +59,7 @@
                 #:rest arguments)
   "Return a bag for NAME."
   (define private-keywords
-    '(#:source #:target #:perl #:inputs #:native-inputs))
+    '(#:target #:perl #:inputs #:native-inputs))
 
   (and (not target)                               ;XXX: no cross-compilation
        (bag
@@ -76,8 +78,8 @@
          (build perl-build)
          (arguments (strip-keyword-arguments private-keywords arguments)))))
 
-(define* (perl-build store name inputs
-                     #:key
+(define* (perl-build name inputs
+                     #:key source
                      (search-paths '())
                      (tests? #t)
                      (parallel-build? #t)
@@ -95,46 +97,34 @@
                                 (guix build utils))))
   "Build SOURCE using PERL, and with INPUTS.  This assumes that SOURCE
 provides a `Makefile.PL' file as its build system."
-  (define builder
-    `(begin
-       (use-modules ,@modules)
-       (perl-build #:name ,name
-                   #:source ,(match (assoc-ref inputs "source")
-                               (((? derivation? source))
-                                (derivation->output-path source))
-                               ((source)
-                                source)
-                               (source
-                                source))
-                   #:search-paths ',(map search-path-specification->sexp
-                                         search-paths)
-                   #:make-maker? ,make-maker?
-                   #:make-maker-flags ,make-maker-flags
-                   #:module-build-flags ,module-build-flags
-                   #:phases ,phases
-                   #:system ,system
-                   #:test-target "test"
-                   #:tests? ,tests?
-                   #:parallel-build? ,parallel-build?
-                   #:parallel-tests? ,parallel-tests?
-                   #:outputs %outputs
-                   #:inputs %build-inputs)))
+  (define build
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
 
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
+          #$(with-build-variables inputs outputs
+              #~(perl-build #:name #$name
+                            #:source #+source
+                            #:search-paths '#$(map search-path-specification->sexp
+                                                   search-paths)
+                            #:make-maker? #$make-maker?
+                            #:make-maker-flags #$make-maker-flags
+                            #:module-build-flags #$module-build-flags
+                            #:phases #$phases
+                            #:system #$system
+                            #:test-target "test"
+                            #:tests? #$tests?
+                            #:parallel-build? #$parallel-build?
+                            #:parallel-tests? #$parallel-tests?
+                            #:outputs %outputs
+                            #:inputs %build-inputs)))))
 
-  (build-expression->derivation store name builder
-                                #:system system
-                                #:inputs inputs
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build))
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name build
+                      #:system system
+                      #:target #f
+                      #:guile-for-build guile)))
 
 (define perl-build-system
   (build-system
diff --git a/guix/build-system/python.scm b/guix/build-system/python.scm
index 9f3159a960..018fda9b20 100644
--- a/guix/build-system/python.scm
+++ b/guix/build-system/python.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2013, 2014, 2015, 2016, 2017 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2013, 2014, 2015, 2016, 2017, 2021 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2013 Andreas Enge <andreas@enge.fr>
 ;;; Copyright © 2013 Nikita Karetnikov <nikita@karetnikov.org>
 ;;; Copyright © 2021 Lars-Dominik Braun <lars@6xq.net>
@@ -25,6 +25,8 @@
   #:use-module (guix store)
   #:use-module (guix utils)
   #:use-module (guix memoization)
+  #:use-module (guix gexp)
+  #:use-module (guix monads)
   #:use-module (guix packages)
   #:use-module (guix derivations)
   #:use-module (guix search-paths)
@@ -147,7 +149,7 @@ pre-defined variants."
                 #:rest arguments)
   "Return a bag for NAME."
   (define private-keywords
-    '(#:source #:target #:python #:inputs #:native-inputs))
+    '(#:target #:python #:inputs #:native-inputs))
 
   (and (not target)                               ;XXX: no cross-compilation
        (bag
@@ -167,8 +169,8 @@ pre-defined variants."
          (build python-build)
          (arguments (strip-keyword-arguments private-keywords arguments)))))
 
-(define* (python-build store name inputs
-                       #:key
+(define* (python-build name inputs
+                       #:key source
                        (tests? #t)
                        (test-target "test")
                        (use-setuptools? #t)
@@ -184,43 +186,32 @@ pre-defined variants."
                                   (guix build utils))))
   "Build SOURCE using PYTHON, and with INPUTS.  This assumes that SOURCE
 provides a 'setup.py' file as its build system."
-  (define builder
-    `(begin
-       (use-modules ,@modules)
-       (python-build #:name ,name
-                     #:source ,(match (assoc-ref inputs "source")
-                                 (((? derivation? source))
-                                  (derivation->output-path source))
-                                 ((source)
-                                  source)
-                                 (source
-                                  source))
-                     #:configure-flags ,configure-flags
-                     #:system ,system
-                     #:test-target ,test-target
-                     #:tests? ,tests?
-                     #:use-setuptools? ,use-setuptools?
-                     #:phases ,phases
-                     #:outputs %outputs
-                     #:search-paths ',(map search-path-specification->sexp
-                                           search-paths)
-                     #:inputs %build-inputs)))
-
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
-
-  (build-expression->derivation store name builder
-                                #:inputs inputs
-                                #:system system
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build))
+  (define build
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
+
+          #$(with-build-variables inputs outputs
+              #~(python-build #:name #$name
+                              #:source #+source
+                              #:configure-flags #$configure-flags
+                              #:use-setuptools? #$use-setuptools?
+                              #:system #$system
+                              #:test-target #$test-target
+                              #:tests? #$tests?
+                              #:phases #$phases
+                              #:outputs %outputs
+                              #:search-paths '#$(map search-path-specification->sexp
+                                                     search-paths)
+                              #:inputs %build-inputs)))))
+
+
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name build
+                      #:system system
+                      #:target #f
+                      #:guile-for-build guile)))
 
 (define python-build-system
   (build-system
diff --git a/guix/build-system/qt.scm b/guix/build-system/qt.scm
index 118022ec45..7df431a68d 100644
--- a/guix/build-system/qt.scm
+++ b/guix/build-system/qt.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2013, 2014, 2015 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2013, 2014, 2015, 2021 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2013 Cyril Roelandt <tipecaml@gmail.com>
 ;;; Copyright © 2017 Ricardo Wurmus <rekado@elephly.net>
 ;;; Copyright © 2019 Hartmut Goebel <h.goebel@crazy-compilers.com>
@@ -22,7 +22,8 @@
 (define-module (guix build-system qt)
   #:use-module (guix store)
   #:use-module (guix utils)
-  #:use-module (guix derivations)
+  #:use-module (guix gexp)
+  #:use-module (guix monads)
   #:use-module (guix search-paths)
   #:use-module (guix build-system)
   #:use-module (guix build-system cmake)
@@ -71,7 +72,7 @@
                 #:rest arguments)
   "Return a bag for NAME."
   (define private-keywords
-    `(#:source #:cmake #:inputs #:native-inputs #:outputs
+    `(#:cmake #:inputs #:native-inputs #:outputs
       ,@(if target '() '(#:target))))
 
   (bag
@@ -105,8 +106,9 @@
     (arguments (strip-keyword-arguments private-keywords arguments))))
 
 
-(define* (qt-build store name inputs
-                   #:key (guile #f)
+(define* (qt-build name inputs
+                   #:key
+                   source (guile #f)
                    (outputs '("out")) (configure-flags ''())
                    (search-paths '())
                    (make-flags ''())
@@ -131,60 +133,46 @@
   "Build SOURCE using CMAKE, and with INPUTS. This assumes that SOURCE
 provides a 'CMakeLists.txt' file as its build system."
   (define builder
-    `(begin
-       (use-modules ,@modules)
-       (qt-build #:source ,(match (assoc-ref inputs "source")
-                                  (((? derivation? source))
-                                   (derivation->output-path source))
-                                  ((source)
-                                   source)
-                                  (source
-                                   source))
-                 #:system ,system
-                 #:outputs %outputs
-                 #:inputs %build-inputs
-                 #:search-paths ',(map search-path-specification->sexp
-                                       search-paths)
-                 #:phases ,phases
-                 #:qt-wrap-excluded-outputs ,qt-wrap-excluded-outputs
-                 #:configure-flags ,configure-flags
-                 #:make-flags ,make-flags
-                 #:out-of-source? ,out-of-source?
-                 #:build-type ,build-type
-                 #:tests? ,tests?
-                 #:test-target ,test-target
-                 #:parallel-build? ,parallel-build?
-                 #:parallel-tests? ,parallel-tests?
-                 #:validate-runpath? ,validate-runpath?
-                 #:patch-shebangs? ,patch-shebangs?
-                 #:strip-binaries? ,strip-binaries?
-                 #:strip-flags ,strip-flags
-                 #:strip-directories ,strip-directories)))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
+          (qt-build #:source #+source
+                    #:system #$system
+                    #:outputs #$(outputs->gexp outputs)
+                    #:inputs #$(input-tuples->gexp inputs)
+                    #:search-paths '#$(map search-path-specification->sexp
+                                           search-paths)
+                    #:phases #$phases
+                    #:qt-wrap-excluded-outputs #$qt-wrap-excluded-outputs
+                    #:configure-flags #$configure-flags
+                    #:make-flags #$make-flags
+                    #:out-of-source? #$out-of-source?
+                    #:build-type #$build-type
+                    #:tests? #$tests?
+                    #:test-target #$test-target
+                    #:parallel-build? #$parallel-build?
+                    #:parallel-tests? #$parallel-tests?
+                    #:validate-runpath? #$validate-runpath?
+                    #:patch-shebangs? #$patch-shebangs?
+                    #:strip-binaries? #$strip-binaries?
+                    #:strip-flags #$strip-flags
+                    #:strip-directories #$strip-directories))))
 
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
-
-  (build-expression->derivation store name builder
-                                #:system system
-                                #:inputs inputs
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build))
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name builder
+                      #:system system
+                      #:guile-for-build guile)))
 
 
 ;;;
 ;;; Cross-compilation.
 ;;;
 
-(define* (qt-cross-build store name
+(define* (qt-cross-build name
                          #:key
-                         target native-drvs target-drvs
+                         source target
+                         build-inputs target-inputs host-inputs
                          (guile #f)
                          (outputs '("out"))
                          (configure-flags ''())
@@ -193,7 +181,7 @@ provides a 'CMakeLists.txt' file as its build system."
                          (make-flags ''())
                          (out-of-source? #t)
                          (build-type "RelWithDebInfo")
-                         (tests? #f) ; nothing can be done
+                         (tests? #f)              ; nothing can be done
                          (test-target "test")
                          (parallel-build? #t) (parallel-tests? #f)
                          (validate-runpath? #t)
@@ -214,77 +202,52 @@ provides a 'CMakeLists.txt' file as its build system."
 with INPUTS.  This assumes that SOURCE provides a 'CMakeLists.txt' file as its
 build system."
   (define builder
-    `(begin
-       (use-modules ,@modules)
-       (let ()
-         (define %build-host-inputs
-           ',(map (match-lambda
-                    ((name (? derivation? drv) sub ...)
-                     `(,name . ,(apply derivation->output-path drv sub)))
-                    ((name path)
-                     `(,name . ,path)))
-                  native-drvs))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
+
+          (define %build-host-inputs
+            #+(input-tuples->gexp build-inputs))
 
-         (define %build-target-inputs
-           ',(map (match-lambda
-                    ((name (? derivation? drv) sub ...)
-                     `(,name . ,(apply derivation->output-path drv sub)))
-                    ((name (? package? pkg) sub ...)
-                     (let ((drv (package-cross-derivation store pkg
-                                                          target system)))
-                       `(,name . ,(apply derivation->output-path drv sub))))
-                    ((name path)
-                     `(,name . ,path)))
-                  target-drvs))
+          (define %build-target-inputs
+            (append #$(input-tuples->gexp host-inputs)
+                    #+(input-tuples->gexp target-inputs)))
 
-         (qt-build #:source ,(match (assoc-ref native-drvs "source")
-                                    (((? derivation? source))
-                                     (derivation->output-path source))
-                                    ((source)
-                                     source)
-                                    (source
-                                     source))
-                   #:system ,system
-                   #:build ,build
-                   #:target ,target
-                   #:outputs %outputs
-                   #:inputs %build-target-inputs
-                   #:native-inputs %build-host-inputs
-                   #:search-paths ',(map search-path-specification->sexp
-                                         search-paths)
-                   #:native-search-paths ',(map
-                                            search-path-specification->sexp
-                                            native-search-paths)
-                   #:phases ,phases
-                   #:configure-flags ,configure-flags
-                   #:make-flags ,make-flags
-                   #:out-of-source? ,out-of-source?
-                   #:build-type ,build-type
-                   #:tests? ,tests?
-                   #:test-target ,test-target
-                   #:parallel-build? ,parallel-build?
-                   #:parallel-tests? ,parallel-tests?
-                   #:validate-runpath? ,validate-runpath?
-                   #:patch-shebangs? ,patch-shebangs?
-                   #:strip-binaries? ,strip-binaries?
-                   #:strip-flags ,strip-flags
-                   #:strip-directories ,strip-directories))))
+          (define %outputs
+            #$(outputs->gexp outputs))
 
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                               ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
+          (qt-build #:source #+source
+                    #:system #$system
+                    #:build #$build
+                    #:target #$target
+                    #:outputs %outputs
+                    #:inputs %build-target-inputs
+                    #:native-inputs %build-host-inputs
+                    #:search-paths '#$(map search-path-specification->sexp
+                                           search-paths)
+                    #:native-search-paths '#$(map
+                                              search-path-specification->sexp
+                                              native-search-paths)
+                    #:phases #$phases
+                    #:configure-flags #$configure-flags
+                    #:make-flags #$make-flags
+                    #:out-of-source? #$out-of-source?
+                    #:build-type #$build-type
+                    #:tests? #$tests?
+                    #:test-target #$test-target
+                    #:parallel-build? #$parallel-build?
+                    #:parallel-tests? #$parallel-tests?
+                    #:validate-runpath? #$validate-runpath?
+                    #:patch-shebangs? #$patch-shebangs?
+                    #:strip-binaries? #$strip-binaries?
+                    #:strip-flags #$strip-flags
+                    #:strip-directories #$strip-directories))))
 
-  (build-expression->derivation store name builder
-                                #:system system
-                                #:inputs (append native-drvs target-drvs)
-                                #:outputs outputs
-                                #:modules imported-modules
-                                #:guile-for-build guile-for-build))
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name builder
+                      #:system system
+                      #:guile-for-build guile)))
 
 (define qt-build-system
   (build-system
diff --git a/guix/build-system/r.scm b/guix/build-system/r.scm
index e2bf41f18d..12b7df66a6 100644
--- a/guix/build-system/r.scm
+++ b/guix/build-system/r.scm
@@ -1,5 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2015, 2017, 2018, 2019, 2020 Ricardo Wurmus <rekado@elephly.net>
+;;; Copyright © 2021 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -20,7 +21,8 @@
   #:use-module (guix store)
   #:use-module (guix utils)
   #:use-module (guix packages)
-  #:use-module (guix derivations)
+  #:use-module (guix gexp)
+  #:use-module (guix monads)
   #:use-module (guix search-paths)
   #:use-module (guix build-system)
   #:use-module (guix build-system gnu)
@@ -82,7 +84,7 @@ release corresponding to NAME and VERSION."
                 #:rest arguments)
   "Return a bag for NAME."
   (define private-keywords
-    '(#:source #:target #:r #:inputs #:native-inputs))
+    '(#:target #:r #:inputs #:native-inputs))
 
   (and (not target)                               ;XXX: no cross-compilation
        (bag
@@ -101,8 +103,9 @@ release corresponding to NAME and VERSION."
          (build r-build)
          (arguments (strip-keyword-arguments private-keywords arguments)))))
 
-(define* (r-build store name inputs
+(define* (r-build name inputs
                   #:key
+                  source
                   (tests? #t)
                   (test-target "tests")
                   (configure-flags ''())
@@ -118,42 +121,27 @@ release corresponding to NAME and VERSION."
                              (guix build utils))))
   "Build SOURCE with INPUTS."
   (define builder
-    `(begin
-       (use-modules ,@modules)
-       (r-build #:name ,name
-                #:source ,(match (assoc-ref inputs "source")
-                            (((? derivation? source))
-                             (derivation->output-path source))
-                            ((source)
-                             source)
-                            (source
-                             source))
-                #:configure-flags ,configure-flags
-                #:system ,system
-                #:tests? ,tests?
-                #:test-target ,test-target
-                #:phases ,phases
-                #:outputs %outputs
-                #:search-paths ',(map search-path-specification->sexp
-                                      search-paths)
-                #:inputs %build-inputs)))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
+          (r-build #:name #$name
+                   #:source #+source
+                   #:configure-flags #$configure-flags
+                   #:system #$system
+                   #:tests? #$tests?
+                   #:test-target #$test-target
+                   #:phases #$phases
+                   #:outputs #$(outputs->gexp outputs)
+                   #:search-paths '#$(map search-path-specification->sexp
+                                          search-paths)
+                   #:inputs #$(input-tuples->gexp inputs)))))
 
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
-
-  (build-expression->derivation store name builder
-                                #:inputs inputs
-                                #:system system
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build
-                                #:substitutable? substitutable?))
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name builder
+                      #:system system
+                      #:guile-for-build guile
+                      #:substitutable? substitutable?)))
 
 (define r-build-system
   (build-system
diff --git a/guix/build-system/rakudo.scm b/guix/build-system/rakudo.scm
index a02e2bad3a..eab41e2cb9 100644
--- a/guix/build-system/rakudo.scm
+++ b/guix/build-system/rakudo.scm
@@ -1,5 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2019 Efraim Flashner <efraim@flashner.co.il>
+;;; Copyright © 2021 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -19,7 +20,8 @@
 (define-module (guix build-system rakudo)
   #:use-module (guix store)
   #:use-module (guix utils)
-  #:use-module (guix derivations)
+  #:use-module (guix gexp)
+  #:use-module (guix monads)
   #:use-module (guix search-paths)
   #:use-module (guix build-system)
   #:use-module (guix build-system gnu)
@@ -71,7 +73,7 @@
                 #:rest arguments)
   "Return a bag for NAME."
   (define private-keywords
-    '(#:source #:target #:rakudo #:prove6 #:zef #:inputs #:native-inputs))
+    '(#:target #:rakudo #:prove6 #:zef #:inputs #:native-inputs))
 
   (and (not target)                               ;XXX: no cross-compilation
        (bag
@@ -96,8 +98,9 @@
          (build rakudo-build)
          (arguments (strip-keyword-arguments private-keywords arguments)))))
 
-(define* (rakudo-build store name inputs
+(define* (rakudo-build name inputs
                        #:key
+                       source
                        (search-paths '())
                        (tests? #t)
                        (phases '(@ (guix build rakudo-build-system)
@@ -112,39 +115,24 @@
                                   (guix build utils))))
   "Build SOURCE using PERL6, and with INPUTS."
   (define builder
-    `(begin
-       (use-modules ,@modules)
-       (rakudo-build #:name ,name
-                     #:source ,(match (assoc-ref inputs "source")
-                                      (((? derivation? source))
-                                       (derivation->output-path source))
-                                      ((source)
-                                       source)
-                                      (source
-                                        source))
-                     #:search-paths ',(map search-path-specification->sexp
-                                           search-paths)
-                     #:phases ,phases
-                     #:system ,system
-                     #:tests? ,tests?
-                     #:outputs %outputs
-                     #:inputs %build-inputs)))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
+          (rakudo-build #:name #$name
+                        #:source #+source
+                        #:search-paths '#$(map search-path-specification->sexp
+                                               search-paths)
+                        #:phases #$phases
+                        #:system #$system
+                        #:tests? #$tests?
+                        #:outputs #$(outputs->gexp outputs)
+                        #:inputs #$(input-tuples->gexp inputs)))))
 
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
-
-  (build-expression->derivation store name builder
-                                #:system system
-                                #:inputs inputs
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build))
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name builder
+                      #:system system
+                      #:guile-for-build guile)))
 
 (define rakudo-build-system
   (build-system
diff --git a/guix/build-system/renpy.scm b/guix/build-system/renpy.scm
index 35edc0056d..5c65f55455 100644
--- a/guix/build-system/renpy.scm
+++ b/guix/build-system/renpy.scm
@@ -1,5 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2021 Leo Prikler <leo.prikler@student.tugraz.at>
+;;; Copyright © 2021 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -21,7 +22,8 @@
   #:use-module (guix utils)
   #:use-module (guix memoization)
   #:use-module (guix packages)
-  #:use-module (guix derivations)
+  #:use-module (guix gexp)
+  #:use-module (guix monads)
   #:use-module (guix search-paths)
   #:use-module (guix build-system)
   #:use-module (guix build-system gnu)
@@ -53,7 +55,7 @@
                 #:rest arguments)
   "Return a bag for NAME."
   (define private-keywords
-    '(#:source #:target #:renpy #:inputs #:native-inputs))
+    '(#:target #:renpy #:inputs #:native-inputs))
 
   (and (not target)                               ;XXX: no cross-compilation
        (bag
@@ -72,57 +74,43 @@
          (build renpy-build)
          (arguments (strip-keyword-arguments private-keywords arguments)))))
 
-(define* (renpy-build store name inputs
-                       #:key
-                       (phases '(@ (guix build renpy-build-system)
-                                   %standard-phases))
-                       (configure-flags ''())
-                       (outputs '("out"))
-                       (output "out")
-                       (game "game")
-                       (search-paths '())
-                       (system (%current-system))
-                       (guile #f)
-                       (imported-modules %renpy-build-system-modules)
-                       (modules '((guix build renpy-build-system)
-                                  (guix build utils))))
+(define* (renpy-build name inputs
+                      #:key
+                      source
+                      (phases '(@ (guix build renpy-build-system)
+                                  %standard-phases))
+                      (configure-flags ''())
+                      (outputs '("out"))
+                      (output "out")
+                      (game "game")
+                      (search-paths '())
+                      (system (%current-system))
+                      (guile #f)
+                      (imported-modules %renpy-build-system-modules)
+                      (modules '((guix build renpy-build-system)
+                                 (guix build utils))))
   "Build SOURCE using RENPY, and with INPUTS."
   (define builder
-    `(begin
-       (use-modules ,@modules)
-       (renpy-build #:name ,name
-                    #:source ,(match (assoc-ref inputs "source")
-                                (((? derivation? source))
-                                 (derivation->output-path source))
-                                ((source)
-                                 source)
-                                (source
-                                 source))
-                     #:configure-flags ,configure-flags
-                     #:system ,system
-                     #:phases ,phases
-                     #:outputs %outputs
-                     #:output ,output
-                     #:game ,game
-                     #:search-paths ',(map search-path-specification->sexp
-                                           search-paths)
-                     #:inputs %build-inputs)))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
+          (renpy-build #:name #$name
+                       #:source #+source
+                       #:configure-flags #$configure-flags
+                       #:system #$system
+                       #:phases #$phases
+                       #:outputs #$(outputs->gexp outputs)
+                       #:output #$output
+                       #:game #$game
+                       #:search-paths '#$(map search-path-specification->sexp
+                                              search-paths)
+                       #:inputs #$(input-tuples->gexp inputs)))))
 
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
-
-  (build-expression->derivation store name builder
-                                #:inputs inputs
-                                #:system system
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build))
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name builder
+                      #:system system
+                      #:guile-for-build guile)))
 
 (define renpy-build-system
   (build-system
diff --git a/guix/build-system/ruby.scm b/guix/build-system/ruby.scm
index 8142e8551a..8b02e0ff52 100644
--- a/guix/build-system/ruby.scm
+++ b/guix/build-system/ruby.scm
@@ -1,6 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2014 David Thompson <davet@gnu.org>
-;;; Copyright © 2014, 2015 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2014, 2015, 2021 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -20,6 +20,8 @@
 (define-module (guix build-system ruby)
   #:use-module (guix store)
   #:use-module (guix utils)
+  #:use-module (guix gexp)
+  #:use-module (guix monads)
   #:use-module (guix packages)
   #:use-module (guix derivations)
   #:use-module (guix search-paths)
@@ -54,7 +56,7 @@ NAME and VERSION."
                 #:rest arguments)
   "Return a bag for NAME."
   (define private-keywords
-    '(#:source #:target #:ruby #:inputs #:native-inputs))
+    '(#:target #:ruby #:inputs #:native-inputs))
 
   (and (not target)                    ;XXX: no cross-compilation
        (bag
@@ -73,8 +75,8 @@ NAME and VERSION."
          (build ruby-build)
          (arguments (strip-keyword-arguments private-keywords arguments)))))
 
-(define* (ruby-build store name inputs
-                     #:key
+(define* (ruby-build name inputs
+                     #:key source
                      (gem-flags ''())
                      (test-target "test")
                      (tests? #t)
@@ -88,42 +90,30 @@ NAME and VERSION."
                      (modules '((guix build ruby-build-system)
                                 (guix build utils))))
   "Build SOURCE using RUBY and INPUTS."
-  (define builder
-    `(begin
-       (use-modules ,@modules)
-       (ruby-build #:name ,name
-                   #:source ,(match (assoc-ref inputs "source")
-                               (((? derivation? source))
-                                (derivation->output-path source))
-                               ((source)
-                                source)
-                               (source
-                                source))
-                   #:system ,system
-                   #:gem-flags ,gem-flags
-                   #:test-target ,test-target
-                   #:tests? ,tests?
-                   #:phases ,phases
-                   #:outputs %outputs
-                   #:search-paths ',(map search-path-specification->sexp
-                                         search-paths)
-                   #:inputs %build-inputs)))
+  (define build
+    #~(begin
+        (use-modules #$@modules)
 
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
+        #$(with-build-variables inputs outputs
+            #~(ruby-build #:name #$name
+                          #:source #+source
+                          #:system #$system
+                          #:gem-flags #$gem-flags
+                          #:test-target #$test-target
+                          #:tests? #$tests?
+                          #:phases #$phases
+                          #:outputs %outputs
+                          #:search-paths '#$(map search-path-specification->sexp
+                                                 search-paths)
+                          #:inputs %build-inputs))))
 
-  (build-expression->derivation store name builder
-                                #:inputs inputs
-                                #:system system
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build))
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name build
+                      #:system system
+                      #:target #f
+                      #:modules imported-modules
+                      #:guile-for-build guile)))
 
 (define ruby-build-system
   (build-system
diff --git a/guix/build-system/scons.scm b/guix/build-system/scons.scm
index aad455c419..6af6998d26 100644
--- a/guix/build-system/scons.scm
+++ b/guix/build-system/scons.scm
@@ -1,5 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2017 Arun Isaac <arunisaac@systemreboot.net>
+;;; Copyright © 2021 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -19,7 +20,8 @@
 (define-module (guix build-system scons)
   #:use-module (guix utils)
   #:use-module (guix packages)
-  #:use-module (guix derivations)
+  #:use-module (guix monads)
+  #:use-module (guix gexp)
   #:use-module (guix search-paths)
   #:use-module (guix build-system)
   #:use-module (guix build-system gnu)
@@ -53,7 +55,7 @@
                 #:rest arguments)
   "Return a bag for NAME."
   (define private-keywords
-    '(#:source #:target #:scons #:inputs #:native-inputs))
+    '(#:target #:scons #:inputs #:native-inputs))
 
   (and (not target)                               ;XXX: no cross-compilation
        (bag
@@ -72,8 +74,9 @@
          (build scons-build)
          (arguments (strip-keyword-arguments private-keywords arguments)))))
 
-(define* (scons-build store name inputs
+(define* (scons-build name inputs
                       #:key
+                      (source #f)
                       (tests? #t)
                       (scons-flags ''())
                       (build-targets ''())
@@ -91,43 +94,33 @@
   "Build SOURCE using SCons, and with INPUTS.  This assumes that SOURCE
 provides a 'SConstruct' file as its build system."
   (define builder
-    `(begin
-       (use-modules ,@modules)
-       (scons-build #:name ,name
-                    #:source ,(match (assoc-ref inputs "source")
-                                (((? derivation? source))
-                                 (derivation->output-path source))
-                                ((source)
-                                 source)
-                                (source
-                                 source))
-                    #:scons-flags ,scons-flags
-                    #:system ,system
-                    #:build-targets ,build-targets
-                    #:test-target ,test-target
-                    #:tests? ,tests?
-                    #:install-targets ,install-targets
-                    #:phases ,phases
-                    #:outputs %outputs
-                    #:search-paths ',(map search-path-specification->sexp
-                                          search-paths)
-                    #:inputs %build-inputs)))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
 
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
+          (scons-build #:name ,name
+                       #:source #+source
+                       #:scons-flags #$scons-flags
+                       #:system #$system
+                       #:build-targets #$build-targets
+                       #:test-target #$test-target
+                       #:tests? #$tests?
+                       #:install-targets #$install-targets
+                       #:phases #$phases
+                       #:outputs (list #$@(map (lambda (name)
+                                                 #~(cons #$name
+                                                         (ungexp output name)))
+                                               outputs))
+                       #:inputs (map (lambda (tuple)
+                                       (apply cons tuple))
+                                     '#$inputs)
+                       #:search-paths '#$(map search-path-specification->sexp
+                                              search-paths)))))
 
-  (build-expression->derivation store name builder
-                                #:inputs inputs
-                                #:system system
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build))
+  (gexp->derivation name builder
+                    #:system system
+                    #:target #f
+                    #:guile-for-build guile))
 
 (define scons-build-system
   (build-system
diff --git a/guix/build-system/texlive.scm b/guix/build-system/texlive.scm
index a8545757be..0a69d1f328 100644
--- a/guix/build-system/texlive.scm
+++ b/guix/build-system/texlive.scm
@@ -1,5 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2017 Ricardo Wurmus <rekado@elephly.net>
+;;; Copyright © 2021 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -20,7 +21,8 @@
   #:use-module (guix store)
   #:use-module (guix utils)
   #:use-module (guix packages)
-  #:use-module (guix derivations)
+  #:use-module (guix monads)
+  #:use-module (guix gexp)
   #:use-module (guix search-paths)
   #:use-module (guix build-system)
   #:use-module (guix build-system gnu)
@@ -100,7 +102,7 @@ level package ID."
                 #:rest arguments)
   "Return a bag for NAME."
   (define private-keywords
-    '(#:source #:target #:inputs #:native-inputs
+    '(#:target #:inputs #:native-inputs
       #:texlive-latex-base #:texlive-bin))
 
   (bag
@@ -120,8 +122,9 @@ level package ID."
     (build texlive-build)
     (arguments (strip-keyword-arguments private-keywords arguments))))
 
-(define* (texlive-build store name inputs
+(define* (texlive-build name inputs
                         #:key
+                        source
                         (tests? #f)
                         tex-directory
                         (build-targets #f)
@@ -139,43 +142,31 @@ level package ID."
                                    (guix build utils))))
   "Build SOURCE with INPUTS."
   (define builder
-    `(begin
-       (use-modules ,@modules)
-       (texlive-build #:name ,name
-                      #:source ,(match (assoc-ref inputs "source")
-                                       (((? derivation? source))
-                                        (derivation->output-path source))
-                                       ((source)
-                                        source)
-                                       (source
-                                        source))
-                      #:tex-directory ,tex-directory
-                      #:build-targets ,build-targets
-                      #:tex-format ,tex-format
-                      #:system ,system
-                      #:tests? ,tests?
-                      #:phases ,phases
-                      #:outputs %outputs
-                      #:search-paths ',(map search-path-specification->sexp
-                                            search-paths)
-                      #:inputs %build-inputs)))
-
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                               ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
-
-  (build-expression->derivation store name builder
-                                #:inputs inputs
-                                #:system system
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build
-                                #:substitutable? substitutable?))
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@modules)
+          (texlive-build #:name #$name
+                         #:source #+source
+                         #:tex-directory #$tex-directory
+                         #:build-targets #$build-targets
+                         #:tex-format #$tex-format
+                         #:system #$system
+                         #:tests? #$tests?
+                         #:phases #$phases
+                         #:outputs (list #$@(map (lambda (name)
+                                                   #~(cons #$name
+                                                           (ungexp output name)))
+                                                 outputs))
+                         #:inputs (map (lambda (tuple)
+                                         (apply cons tuple))
+                                       '#$inputs)
+                         #:search-paths '#$(map search-path-specification->sexp
+                                                search-paths)))))
+
+  (gexp->derivation name builder
+                    #:system system
+                    #:target #f
+                    #:substitutable? substitutable?))
 
 (define texlive-build-system
   (build-system
diff --git a/guix/build-system/trivial.scm b/guix/build-system/trivial.scm
index b50ef7cd92..0f89524231 100644
--- a/guix/build-system/trivial.scm
+++ b/guix/build-system/trivial.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2012, 2013, 2014, 2018 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2012, 2013, 2014, 2015, 2018, 2021 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -19,24 +19,16 @@
 (define-module (guix build-system trivial)
   #:use-module (guix store)
   #:use-module (guix utils)
-  #:use-module (guix derivations)
+  #:use-module (guix gexp)
+  #:use-module (guix monads)
   #:use-module (guix packages)
   #:use-module (guix build-system)
   #:use-module (ice-9 match)
   #:export (trivial-build-system))
 
-(define (guile-for-build store guile system)
-  (match guile
-    ((? package?)
-     (package-derivation store guile system #:graft? #f))
-    (#f                                         ; the default
-     (let* ((distro (resolve-interface '(gnu packages commencement)))
-            (guile  (module-ref distro 'guile-final)))
-       (package-derivation store guile system #:graft? #f)))))
-
 (define* (lower name
                 #:key source inputs native-inputs outputs system target
-                guile builder modules allowed-references)
+                guile builder (modules '()) allowed-references)
   "Return a bag for NAME."
   (bag
     (name name)
@@ -54,65 +46,42 @@
                  #:modules ,modules
                  #:allowed-references ,allowed-references))))
 
-(define* (trivial-build store name inputs
+(define* (trivial-build name inputs
                         #:key
-                        outputs guile system builder (modules '())
+                        outputs guile
+                        system builder (modules '())
                         search-paths allowed-references)
   "Run build expression BUILDER, an expression, for SYSTEM.  SOURCE is
 ignored."
-  (define canonicalize-reference
-    (match-lambda
-     ((? package? p)
-      (derivation->output-path (package-derivation store p system
-                                                   #:graft? #f)))
-     (((? package? p) output)
-      (derivation->output-path (package-derivation store p system
-                                                   #:graft? #f)
-                               output))
-     ((? string? output)
-      output)))
-
-  (build-expression->derivation store name builder
-                                #:inputs inputs
-                                #:system system
-                                #:outputs outputs
-                                #:modules modules
-                                #:allowed-references
-                                (and allowed-references
-                                     (map canonicalize-reference
-                                          allowed-references))
-                                #:guile-for-build
-                                (guile-for-build store guile system)))
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name (with-build-variables inputs outputs builder)
+                      #:system system
+                      #:target #f
+                      #:modules modules
+                      #:allowed-references allowed-references
+                      #:guile-for-build guile)))
 
-(define* (trivial-cross-build store name
+(define* (trivial-cross-build name
                               #:key
-                              target native-drvs target-drvs
+                              target
+                              source build-inputs target-inputs host-inputs
                               outputs guile system builder (modules '())
                               search-paths native-search-paths
                               allowed-references)
   "Run build expression BUILDER, an expression, for SYSTEM.  SOURCE is
 ignored."
-  (define canonicalize-reference
-    (match-lambda
-     ((? package? p)
-      (derivation->output-path (package-cross-derivation store p system)))
-     (((? package? p) output)
-      (derivation->output-path (package-cross-derivation store p system)
-                               output))
-     ((? string? output)
-      output)))
-
-  (build-expression->derivation store name builder
-                                #:inputs (append native-drvs target-drvs)
-                                #:system system
-                                #:outputs outputs
-                                #:modules modules
-                                #:allowed-references
-                                (and allowed-references
-                                     (map canonicalize-reference
-                                          allowed-references))
-                                #:guile-for-build
-                                (guile-for-build store guile system)))
+  (mlet %store-monad  ((guile (package->derivation (or guile (default-guile))
+                                                   system #:graft? #f)))
+    (gexp->derivation name (with-build-variables
+                               (append build-inputs target-inputs)
+                               outputs
+                             builder)
+                      #:system system
+                      #:target target
+                      #:modules modules
+                      #:allowed-references allowed-references
+                      #:guile-for-build guile)))
 
 (define trivial-build-system
   (build-system
diff --git a/guix/build-system/waf.scm b/guix/build-system/waf.scm
index 044d2a0829..db604e6ecf 100644
--- a/guix/build-system/waf.scm
+++ b/guix/build-system/waf.scm
@@ -19,6 +19,8 @@
 (define-module (guix build-system waf)
   #:use-module (guix store)
   #:use-module (guix utils)
+  #:use-module (guix gexp)
+  #:use-module (guix monads)
   #:use-module (guix packages)
   #:use-module (guix derivations)
   #:use-module (guix search-paths)
@@ -52,7 +54,7 @@
                 #:rest arguments)
   "Return a bag for NAME."
   (define private-keywords
-    '(#:source #:target #:python #:inputs #:native-inputs))
+    '(#:target #:python #:inputs #:native-inputs))
 
   (and (not target)                               ;XXX: no cross-compilation
        (bag
@@ -71,58 +73,46 @@
          (build waf-build) ; only change compared to 'lower' in python.scm
          (arguments (strip-keyword-arguments private-keywords arguments)))))
 
-(define* (waf-build store name inputs
-                       #:key
-                       (tests? #t)
-                       (test-target "check")
-                       (configure-flags ''())
-                       (phases '(@ (guix build waf-build-system)
-                                   %standard-phases))
-                       (outputs '("out"))
-                       (search-paths '())
-                       (system (%current-system))
-                       (guile #f)
-                       (imported-modules %waf-build-system-modules)
-                       (modules '((guix build waf-build-system)
-                                  (guix build utils))))
+(define* (waf-build name inputs
+                    #:key source
+                    (tests? #t)
+                    (test-target "check")
+                    (configure-flags ''())
+                    (phases '(@ (guix build waf-build-system)
+                                %standard-phases))
+                    (outputs '("out"))
+                    (search-paths '())
+                    (system (%current-system))
+                    (guile #f)
+                    (imported-modules %waf-build-system-modules)
+                    (modules '((guix build waf-build-system)
+                               (guix build utils))))
   "Build SOURCE with INPUTS.  This assumes that SOURCE provides a 'waf' file
 as its build system."
-  (define builder
-    `(begin
-       (use-modules ,@modules)
-       (waf-build #:name ,name
-                  #:source ,(match (assoc-ref inputs "source")
-                              (((? derivation? source))
-                               (derivation->output-path source))
-                              ((source)
-                               source)
-                              (source
-                               source))
-                  #:configure-flags ,configure-flags
-                  #:system ,system
-                  #:test-target ,test-target
-                  #:tests? ,tests?
-                  #:phases ,phases
-                  #:outputs %outputs
-                  #:search-paths ',(map search-path-specification->sexp
-                                        search-paths)
-                  #:inputs %build-inputs)))
+  (define build
+    #~(begin
+        (use-modules #$@modules)
 
-  (define guile-for-build
-    (match guile
-      ((? package?)
-       (package-derivation store guile system #:graft? #f))
-      (#f                                         ; the default
-       (let* ((distro (resolve-interface '(gnu packages commencement)))
-              (guile  (module-ref distro 'guile-final)))
-         (package-derivation store guile system #:graft? #f)))))
+        #$(with-build-variables inputs outputs
+            #~(waf-build #:name #$name
+                         #:source #+source
+                         #:configure-flags #$configure-flags
+                         #:system #$system
+                         #:test-target #$test-target
+                         #:tests? #$tests?
+                         #:phases #$phases
+                         #:outputs %outputs
+                         #:search-paths '#$(map search-path-specification->sexp
+                                                search-paths)
+                         #:inputs %build-inputs))))
 
-  (build-expression->derivation store name builder
-                                #:inputs inputs
-                                #:system system
-                                #:modules imported-modules
-                                #:outputs outputs
-                                #:guile-for-build guile-for-build))
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name build
+                      #:system system
+                      #:target #f
+                      #:modules imported-modules
+                      #:guile-for-build guile)))
 
 (define waf-build-system
   (build-system
diff --git a/guix/gexp.scm b/guix/gexp.scm
index 3817bdd855..a4e6590b52 100644
--- a/guix/gexp.scm
+++ b/guix/gexp.scm
@@ -112,6 +112,7 @@
             mixed-text-file
             file-union
             directory-union
+
             imported-files
             imported-modules
             compiled-modules
diff --git a/guix/packages.scm b/guix/packages.scm
index 56173e1204..6dc652fe7a 100644
--- a/guix/packages.scm
+++ b/guix/packages.scm
@@ -1174,10 +1174,6 @@ matching package and returns a replacement for that package."
 ;;; Package derivations.
 ;;;
 
-(define %derivation-cache
-  ;; Package to derivation-path mapping.
-  (make-weak-key-hash-table 100))
-
 (define (cache! cache package system thunk)
   "Memoize in CACHE the return values of THUNK as the derivation of PACKAGE on
 SYSTEM."
@@ -1209,48 +1205,29 @@ Return the cached result when available."
     ((_ package system body ...)
      (cached (=> %derivation-cache) package system body ...))))
 
-(define* (expand-input store package input system #:optional cross-system)
-  "Expand INPUT, an input tuple, such that it contains only references to
-derivation paths or store paths.  PACKAGE is only used to provide contextual
-information in exceptions."
-  (define (intern file)
-    ;; Add FILE to the store.  Set the `recursive?' bit to #t, so that
-    ;; file permissions are preserved.
-    (add-to-store store (basename file) #t "sha256" file))
-
-  (define derivation
-    (if cross-system
-        (cut package-cross-derivation store <> cross-system system
-             #:graft? #f)
-        (cut package-derivation store <> system #:graft? #f)))
+(define* (expand-input package input #:key native?)
+  "Expand INPUT, an input tuple, to a name/<gexp-input> tuple.  PACKAGE is
+only used to provide contextual information in exceptions."
+  (define (valid? x)
+    (or (package? x) (origin? x) (derivation? x)))
 
   (match input
-    (((? string? name) (? package? package))
-     (list name (derivation package)))
-    (((? string? name) (? package? package)
-      (? string? sub-drv))
-     (list name (derivation package)
-           sub-drv))
-    (((? string? name)
-      (and (? string?) (? derivation-path?) drv))
-     (list name drv))
+    (((? string? name) (? valid? thing))
+     (list name (gexp-input thing #:native? native?)))
+    (((? string? name) (? valid? thing) (? string? output))
+     (list name (gexp-input thing output #:native? native?)))
     (((? string? name)
       (and (? string?) (? file-exists? file)))
      ;; Add FILE to the store.  When FILE is in the sub-directory of a
      ;; store path, it needs to be added anyway, so it can be used as a
      ;; source.
-     (list name (intern file)))
+     (list name (gexp-input (local-file file #:recursive? #t)
+                            #:native? native?)))
     (((? string? name) (? struct? source))
      ;; 'package-source-derivation' calls 'lower-object', which can throw
      ;; '&gexp-input-error'.  However '&gexp-input-error' lacks source
-     ;; location info, so we catch and rethrow here (XXX: not optimal
-     ;; performance-wise).
-     (guard (c ((gexp-input-error? c)
-                (raise (condition
-                        (&package-input-error
-                         (package package)
-                         (input   (gexp-error-invalid-input c)))))))
-       (list name (package-source-derivation store source system))))
+     ;; location info, so we used to catch and rethrow here (FIXME!).
+     (list name (gexp-input source)))
     (x
      (raise (condition (&package-input-error
                         (package package)
@@ -1434,12 +1411,14 @@ TARGET."
 (define (input=? input1 input2)
   "Return true if INPUT1 and INPUT2 are equivalent."
   (match input1
-    ((label1 drv1 . outputs1)
+    ((label1 obj1 . outputs1)
      (match input2
-       ((label2 drv2 . outputs2)
+       ((label2 obj2 . outputs2)
         (and (string=? label1 label2)
              (equal? outputs1 outputs2)
-             (derivation=? drv1 drv2)))))))
+             (or (and (derivation? obj1) (derivation? obj2)
+                      (derivation=? obj1 obj2))
+                 (equal? obj1 obj2))))))))
 
 (define* (bag->derivation store bag
                           #:optional context)
@@ -1450,7 +1429,7 @@ error reporting."
       (bag->cross-derivation store bag)
       (let* ((system     (bag-system bag))
              (inputs     (bag-transitive-inputs bag))
-             (input-drvs (map (cut expand-input store context <> system)
+             (input-drvs (map (cut expand-input context <> #:native? #t)
                               inputs))
              (paths      (delete-duplicates
                           (append-map (match-lambda
@@ -1462,7 +1441,8 @@ error reporting."
         ;; It's possible that INPUTS contains packages that are not 'eq?' but
         ;; that lead to the same derivation.  Delete those duplicates to avoid
         ;; issues down the road, such as duplicate entries in '%build-inputs'.
-        (apply (bag-build bag)
+        ;; TODO: Change to monadic style.
+        (apply (store-lower (bag-build bag))
                store (bag-name bag)
                (delete-duplicates input-drvs input=?)
                #:search-paths paths
@@ -1477,13 +1457,13 @@ This is an internal procedure."
   (let* ((system      (bag-system bag))
          (target      (bag-target bag))
          (host        (bag-transitive-host-inputs bag))
-         (host-drvs   (map (cut expand-input store context <> system target)
+         (host-drvs   (map (cut expand-input context <> #:native? #f)
                            host))
          (target*     (bag-transitive-target-inputs bag))
-         (target-drvs (map (cut expand-input store context <> system)
+         (target-drvs (map (cut expand-input context <> #:native? #t)
                            target*))
          (build       (bag-transitive-build-inputs bag))
-         (build-drvs  (map (cut expand-input store context <> system)
+         (build-drvs  (map (cut expand-input context <> #:native? #t)
                            build))
          (all         (append build target* host))
          (paths       (delete-duplicates
@@ -1500,11 +1480,12 @@ This is an internal procedure."
                                     (_ '()))
                                    all))))
 
-    (apply (bag-build bag)
+    ;; TODO: Change to monadic style.
+    (apply (store-lower (bag-build bag))
            store (bag-name bag)
-           #:native-drvs (delete-duplicates build-drvs input=?)
-           #:target-drvs (delete-duplicates (append host-drvs target-drvs)
-                                            input=?)
+           #:build-inputs (delete-duplicates build-drvs input=?)
+           #:host-inputs (delete-duplicates host-drvs input=?)
+           #:target-inputs (delete-duplicates target-drvs input=?)
            #:search-paths paths
            #:native-search-paths npaths
            #:outputs (bag-outputs bag)
diff --git a/tests/builders.scm b/tests/builders.scm
index 2143c0738b..f36a8c9f59 100644
--- a/tests/builders.scm
+++ b/tests/builders.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2012, 2013, 2014, 2015, 2019 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2012, 2013, 2014, 2015, 2018, 2019 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2021 Lars-Dominik Braun <lars@6xq.net>
 ;;;
 ;;; This file is part of GNU Guix.
diff --git a/tests/lint.scm b/tests/lint.scm
index bd8604f589..47d5701b7d 100644
--- a/tests/lint.scm
+++ b/tests/lint.scm
@@ -1,7 +1,7 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2012, 2013 Cyril Roelandt <tipecaml@gmail.com>
 ;;; Copyright © 2014, 2015, 2016 Eric Bavier <bavier@member.fsf.org>
-;;; Copyright © 2014, 2015, 2016, 2017, 2018, 2019, 2020 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2015, 2016 Mathieu Lirzin <mthl@gnu.org>
 ;;; Copyright © 2016 Hartmut Goebel <h.goebel@crazy-compilers.com>
 ;;; Copyright © 2017 Alex Kost <alezost@gmail.com>
@@ -432,7 +432,7 @@
       (single-lint-warning-message (check-patch-headers pkg)))))
 
 (test-equal "derivation: invalid arguments"
-  "failed to create x86_64-linux derivation: (wrong-type-arg \"map\" \"Wrong type argument: ~S\" (invalid-module) ())"
+  "failed to create x86_64-linux derivation: (match-error \"match\" \"no matching pattern\" invalid-module)"
   (match (let ((pkg (dummy-package "x"
                                    (arguments
                                     '(#:imported-modules (invalid-module))))))
diff --git a/tests/packages.scm b/tests/packages.scm
index ff756c6001..d1dab7d6a5 100644
--- a/tests/packages.scm
+++ b/tests/packages.scm
@@ -868,9 +868,9 @@
                         (system system) (target target)
                         (build-inputs inputs)
                         (build
-                         (lambda* (store name inputs
-                                         #:key outputs system search-paths)
-                           search-paths)))))))
+                         (lambda* (name inputs
+                                        #:key outputs system search-paths)
+                           (abort-to-prompt p search-paths))))))))
          (x (list (search-path-specification
                    (variable "GUILE_LOAD_PATH")
                    (files '("share/guile/site/2.0")))
@@ -1170,11 +1170,11 @@
                        (bag (name name) (system system) (target target)
                             (build-inputs native-inputs)
                             (host-inputs inputs)
-                            (build (lambda* (store name inputs
-                                                   #:key system target
-                                                   #:allow-other-keys)
-                                     (build-expression->derivation
-                                      store "foo" '(mkdir %output))))))))
+                            (build (lambda* (name inputs
+                                                  #:key system target
+                                                  #:allow-other-keys)
+                                     (gexp->derivation "foo"
+                                                       #~(mkdir #$output))))))))
          (bs    (build-system
                   (name 'build-system-without-cross-compilation)
                   (description "Does not support cross compilation.")